[Routing][Tests] Run PhpGeneratorDumper tests in separate process

Due to the single process nature of PHP, the test produces a FatalError with PHP 5.4.29. Adding the PHPUnit annotation to run the tests in separate processes fixes the problem.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | none
| License       | MIT
| Doc PR        | none