Fix some TimerTest flakiness.

There's more flakiness around here, but this is a start. I'm not sure
what we should do with the tests that test how many times a task
has been executed in a given period, so I've left them for now.

Bug: 7766472
Bug: 7745085
Bug: 7740706
Bug: 6896554
Change-Id: I84d8aa036c075024d5cefdad050884762eab50a3