Make usleep longer and simplify assertions
minor #9276 [Stopwatch] Fix tests on Travis (also vagrant) (brikou)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #9276).

Discussion
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[Stopwatch] Fix tests on Travis (also vagrant)

Travis often fails to run tests related to the Stopwatch Component, check it yourself:

- https://travis-ci.org/symfony/symfony/jobs/12340378
- https://travis-ci.org/symfony/symfony/jobs/12365787
- https://travis-ci.org/symfony/symfony/jobs/12365790
- https://travis-ci.org/symfony/symfony/jobs/12367771
- https://travis-ci.org/symfony/symfony/jobs/12367844
- https://travis-ci.org/symfony/symfony/jobs/12368619
- https://travis-ci.org/symfony/symfony/jobs/12371316
- https://travis-ci.org/symfony/symfony/jobs/12375730

This is because Travis is just slow as hell. So I just multiply the used usleep by 10.

Commits
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bf65483 Make usleep longer and simplify assertions