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
[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
[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
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.
renamed variables - making next change more readable
reduced recursion when building DumperPrefixCollection
bug #10215 [Routing] reduced recursion in dumper (arnaud-lb)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #10215).

Discussion
----------

[Routing] reduced recursion in dumper

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | sensiolabs/SensioFrameworkExtraBundle/issues/275
| License       | MIT

This reduces recursion in the route dumper, avoiding issues with xdebug's recursion limit.

Commits
-------

fa6eebc reduced recursion when building DumperPrefixCollection
9278b27 renamed variables - making next change more readable
fixed various inconsistencies
minor #10234 Various minor changes (fabpot)

This PR was merged into the 2.3 branch.

Discussion
----------

Various minor changes

| 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
-------

eb3f6c6 fixed various inconsistencies
Merge branch '2.3' into 2.4

* 2.3:
  fixed various inconsistencies
  reduced recursion when building DumperPrefixCollection
  renamed variables - making next change more readable
  removing dead code.
  [DomCrawler] Fixed filterXPath() chaining
  [DomCrawler] Fixed incorrect handling of image inputs

Conflicts:
	src/Symfony/Component/DomCrawler/Crawler.php
	src/Symfony/Component/EventDispatcher/Tests/EventDispatcherTest.php
	src/Symfony/Component/Form/Extension/DependencyInjection/DependencyInjectionExtension.php
	src/Symfony/Component/Serializer/Tests/Normalizer/CustomNormalizerTest.php
	src/Symfony/Component/Templating/Tests/Loader/CacheLoaderTest.php
	src/Symfony/Component/Templating/Tests/Loader/LoaderTest.php
Merge branch '2.4'

* 2.4:
  fixed various inconsistencies
  reduced recursion when building DumperPrefixCollection
  renamed variables - making next change more readable
  removing dead code.
  [ExpressionLanguage] added some tests for the built-in constant() function
  [ExpressionLanguage] added some documentation about functions
  [DomCrawler] Fixed filterXPath() chaining
  [DomCrawler] Fixed incorrect handling of image inputs