[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.
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
Merge branch '2.4' into 2.5

* 2.4:
  [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
Merge branch '2.5'

* 2.5: (37 commits)
  [Validator] Backported constraint validator tests from 2.5
  [Validator] Backported constraint validator tests from 2.5
  [DIC] Fixed: anonymous services are always private
  Fix toolbar vertical alignment.
  [HttpFoundation] MongoDbSessionHandler supports auto expiry via configurable expiry_field
  [Validator] Fix little typo in ExecutionContextInterface::buildViolation() method comments
  fix dependencies on HttpFoundation component
  [FrameworkBundle] add missing attribute to XSD
  Allow basic auth in url. Improve regex. Add tests.
  fix typos and syntax in Profiler controller method comments
  resolve parameters before the configs are processed
  add symfony/yaml suggestion to composer.json
  [HttpKernel] added an analyze of environment parameters for built-in server.
  remove volatile tests
  [Console] fixed style creation when providing an unknown tag option
  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
  ...

Conflicts:
	src/Symfony/Component/Form/Extension/Validator/Constraints/FormValidator.php
	src/Symfony/Component/HttpKernel/Kernel.php
	src/Symfony/Component/HttpKernel/composer.json
	src/Symfony/Component/Validator/Constraints/AllValidator.php
	src/Symfony/Component/Validator/Constraints/CollectionValidator.php
	src/Symfony/Component/Validator/Constraints/LegacyAllValidator.php
	src/Symfony/Component/Validator/Constraints/LegacyCollectionValidator.php
	src/Symfony/Component/Validator/Tests/Constraints/FileValidatorTest.php