In the light of discovered memory leaks (see nexedi/pygolang!24),
it is better to explicitly make sure that resources allocated by every
test are explicitly released. Even though timers are released
automatically on their expiration, there is generally no guarantee that
the tests will finish after all timers are expired. And even more so for
Ticker - without explicit stop, the ticker continues to be active
forever.

So stop all created timers and tickers where we can in the tests.

/proposed-for-review-on nexedi/pygolang!26