Extract new base test class KernelTestClass

Ideal base class that needs access to a Kernel but not a Client such as
when testing a Command.
[Form] Changed Form::getErrors() to return an iterator and added two optional parameters $deep and $flatten
[Form] Changed the default value of $flatten in Form::getErrors() to true
Include running periods in duration

StopwatchEvent:
  - method getDuration() now includes periods that are not stopped yet

StopwatchEventTest:
  - added testDurationBeforeStop()
Allow retrieving unstopped stopwatch events

Section:
  - added method getEvent()

Stopwatch:
  - added method getEvent()

StopwatchTest:
  - modified testStart() to test for getEvent() method
  - added testUnknownEvent()
[EventDispatcher] simplified code for TraceableEventDispatcher
[DomCrawler] Fixed incorrect handling of image inputs
bug #10205 [DomCrawler] Fixed incorrect handling of image inputs (robbertkl)

This PR was merged into the 2.3 branch.

Discussion
----------

[DomCrawler] Fixed incorrect handling of image inputs

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10204
| License       | MIT
| Doc PR        |

A possible approach to fix #10204, but I'm open to suggestions to fix this another way, as this might not be the most 'elegant' way.

Initially, my thoughts were to create a new DomCrawler\Field\FormField subclass, especially for image inputs. However, this does not solve the problem, because such a FormField would still exist under 1 name in the FormFieldRegistry.

Instead, I've changed it to have 2 separate FormFields instead (which both reference the same input node), with different names (.x and .y) so that both values can be set separately and will both be submitted.

Commits
-------

816cf17 [DomCrawler] Fixed incorrect handling of image inputs
[DomCrawler] Fixed filterXPath() chaining
bug #10207 [DomCrawler] Fixed filterXPath() chaining (robbertkl)

This PR was merged into the 2.3 branch.

Discussion
----------

[DomCrawler] Fixed filterXPath() chaining

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | debatable (see below)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10206
| License       | MIT
| Doc PR        |

As @stof mentions in #10206, each node in the Crawler can belong to a different \DOMDocument. Therefore, I've made each node do its own XPath, relative to itself, and add all the results to a new Crawler. This way, all resulting nodes are still part of their original \DOMDocument and thus can reach all of their parent nodes.

No current tests break on this change. I've added a new test for this case, by checking if the number of parents is correct after obtaining a node through chaining of `filterXPath()`.

Now for BC: I can think of a number of cases where this change would give a different result. However, it's debatable/unclear if:
- the old behavior was a bug in the first place (which would validate this change), or
- the old behavior was intended (which would make this a BC breaking change)

As an example, consider the following HTML:
```html
<div name="a"><div name="b"><div name="c"></div></div></div>
```
What would happen if we run this:
```php
echo $crawler->filterXPath('//div')->filterXPath('div')->filterXPath('div')->attr('name');
```
Aside from breaking reachability of the parent nodes by chaining, with the original code it would echo 'a'.
With this patch it would echo 'c', which, to me, makes more sense.

Commits
-------

43a7716 [DomCrawler] Fixed filterXPath() chaining
feature #10198 [Stopwatch] Allow getting duration of events without calling stop() (jochenvdv)

This PR was merged into the 2.5-dev branch.

Discussion
----------

[Stopwatch] Allow getting duration of events without calling stop()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [#10175](https://github.com/symfony/symfony/issues/10175)
| License       | MIT
| Doc PR        | [#3539](https://github.com/symfony/symfony-docs/pull/3539)

Commits
-------

2efe461 Allow retrieving unstopped stopwatch events
d3d097d Include running periods in duration
feature #10200 [EventDispatcher] simplified code for TraceableEventDispatcher (fabpot)

This PR was merged into the 2.5-dev branch.

Discussion
----------

[EventDispatcher] simplified code for TraceableEventDispatcher

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

42e4c7b [EventDispatcher] simplified code for TraceableEventDispatcher
[3.0][Console] Added isVerbosity* to OutputInterface

Now, only `Output` have theses methods. But in the `Command` class, `execute` receive an instance of `OutputInterface`. So, to avoid coding an implementation, theses method should be added to the interface.
[ExpressionLanguage] added some documentation about functions
[ExpressionLanguage] added some tests for the built-in constant() function
minor #10223 [ExpressionLanguage] added some tests for the built-in constant() function (fabpot)

This PR was merged into the 2.4 branch.

Discussion
----------

[ExpressionLanguage] added some tests for the built-in constant() function

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10216
| License       | MIT
| Doc PR        | n/a

Commits
-------

d8f55f5 [ExpressionLanguage] added some tests for the built-in constant() function
minor #10222 [ExpressionLanguage] added some documentation about functions (fabpot)

This PR was merged into the 2.4 branch.

Discussion
----------

[ExpressionLanguage] added some documentation about functions

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10216
| License       | MIT
| Doc PR        | n/a

This adds documentation about how to define a function in the expression language.

Commits
-------

600d481 [ExpressionLanguage] added some documentation about functions
minor #10214 [3.0][Console] Added isVerbosity* to OutputInterface (lyrixx)

This PR was merged into the 2.5-dev branch.

Discussion
----------

[3.0][Console] Added isVerbosity* to OutputInterface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Now, only `Output` have theses methods. But in the `Command` class, `execute` receive an instance of `OutputInterface`. So, to avoid coding against an implementation, theses method should be added to the interface.

Commits
-------

816b295 [3.0][Console] Added isVerbosity* to OutputInterface
removing dead code.
minor #10219 [EventDispatcher] removing dead code. (inalgnu)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #10219).

Discussion
----------

[EventDispatcher] removing dead code.

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | [n/a]
| License       | MIT
| Doc PR        | [n/a]

Commits
-------

e28395f [EventDispatcher] removing dead code.