Flaky tests in TimerPoolExecutor

[davidor]

There are a couple of tests in TimerPoolExecutor that fail sometimes, although very rarely.

• "TimerPoolExecutor worker garbage collection puts back worker even when task crashes"
• "TimerPoolExecutor worker garbage collection automatically checks in back old workers"

Examples of builds where those tests failed:
https://circleci.com/gh/3scale/APIcast/21029
https://circleci.com/gh/3scale/APIcast/20931
https://circleci.com/gh/3scale/APIcast/20869