[Process] Fix tests when pcntl is not available.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

It's solved the same way in the [AbstractProcessTest](https://github.com/symfony/symfony/blob/3b837dc5c102b8210b7038029416c0524d5f458b/src/Symfony/Component/Process/Tests/AbstractProcessTest.php#L602-L603).