[Process] Avoid failure when calling Process::stop in edge cases

See https://travis-ci.org/symfony/symfony/jobs/20664994 for an example of failure.
[Process] Avoid failure because of a slow process

See example of failure here https://travis-ci.org/symfony/symfony/jobs/20761834.
[Process] Avoid failures because of slow IOs

See example of failure here https://travis-ci.org/symfony/symfony/jobs/20701462
bug #10455 [2.3][Process] Fix random failures in test suite on TravisCI (romainneutron)

This PR was merged into the 2.3 branch.

Discussion
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[2.3][Process] Fix random failures in test suite on TravisCI

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Commits
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783e377 [Process] Avoid failures because of slow IOs
238565e [Process] Avoid failure because of a slow process
173f8c5 [Process] Avoid failure when calling Process::stop in edge cases