This simple PR improves the test suite to use less fragile assumptions for the functional tests and also resets test doubles between test runs to ensure the test suite supports running with --repeat=100.

Also, the old test suite did report an error when using ext-event, both locally and when run on Travis. Note that this project currently only tests against the "default loop", so this wasn't reported here, but actually as part of reactphp/reactphp#396 (comment). Accordingly, it's worth considering running against multiple loop implementations, but I'll try to leave this discussion separated from this PR.

The updated test suite avoids assumptions about the loop tick behavior and the number of tests runs and thus makes these tests less fragile.

Refs reactphp/event-loop#144