[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