[Process] Added process synchronization to the incremental output tests

The tests currently fail from time to time if the executing machine is under
heavy load. This leads to false negatives on Travis CI.

A side effect of the change is that the tests are much faster now.
[Form] allowed CallbackTransformer to use callable
minor #11576 [Form] allowed CallbackTransformer to use callable (issei-m)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #11576).

Discussion
----------

[Form] allowed CallbackTransformer to use callable

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

I want to use the callable(i.e. inner class methods) in CallbackTransformer, but its current constructor is defined argument type hinting for \Closure so i can't.

Commits
-------

e77022b [Form] allowed CallbackTransformer to use callable
fix shell command injection
add way to test command under windows
change command to which available under most unix systems

agree i will change it which is available under suse, ubuntu, debian, fedora, bsd
bug #11598 [Finder] Shell escape and windows support (Gordon Franke, gimler)

This PR was merged into the 2.3 branch.

Discussion
----------

[Finder] Shell escape and windows support

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

Add escaping of command passed to Shell::testCommand().
Fix todo add support for windows.

Commits
-------

c70a226 change command to which available under most unix systems
85e77b1 add way to test command under windows
4fa9288 fix shell command injection
minor #11565 [Process] Added process synchronization to the incremental output tests (webmozart)

This PR was merged into the 2.3 branch.

Discussion
----------

[Process] Added process synchronization to the incremental output tests

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

The tests currently fail from time to time if the executing machine is under
heavy load. This leads to false negatives on Travis CI.

A side effect of the change is that the tests are much faster now.

Commits
-------

6dd3946 [Process] Added process synchronization to the incremental output tests
[HttpKernel] added an analyze of environment parameters for built-in server.
bug #10914 [HttpKernel] added an analyze of environment parameters for built-in server (mauchede)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #10914).

Discussion
----------

[HttpKernel] added an analyze of environment parameters for built-in server

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #10208
| License       | MIT
| Doc PR        | -

With the built-in server, it is not possible to use the external parameters: environment variables are only in `$_ENV`.

Commits
-------

696b978 [HttpKernel] added an analyze of environment parameters for built-in server.
Merge branch '2.3' into 2.4

* 2.3:
  [HttpKernel] added an analyze of environment parameters for built-in server.
  change command to which available under most unix systems
  add way to test command under windows
  fix shell command injection
  [Form] allowed CallbackTransformer to use callable
  [Process] Added process synchronization to the incremental output tests