[Process] Added support for stdout and stderr flush (Issue #7884)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7884
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/2728

**To-do**
- [x] Submit changes to the documentation.
- [x] Fix a test broken on travis.
- [x] Fix mistakes on the documentation.
- [x] Removed flush + get methods.
- [x] Changed tests assert calls.

This PR introduces flushing methods for both stdout and stderr on Process class. The new methods are:
- flushOutput(): clears the output buffer.
- flushErrorOutput(): clears the error output buffer.

Tests for new methods are included on the PR.