TestStart was previously flaky. In approx 100_000 local runs, it failed
about 70% of the time, and has been mentioned as a flaky unit test in
the past.
This flake was due to a race condition with the logic as written and the
go scheduler. UpdateThreshold calls `notifier.Start(events)` in a new Go
Routine, which is not guarunteed to be called immediately.
This meant that if `m.Start()` was called before `notifier.Start()`, the
test would fail, as the notifier would not have been started before the
4 events were processed and lock released.
Here, we update the test to more closely match the intended application
behaviour, and have events passed to the channel when `Start` is called
on the notifier.
This ensures that -Start gets called and additionally validates
that the correct channel is provided to the notifier.
Stop was never called previously, as it only gets called on a subsequent
call to UpdateThreshold. `AnyTimes()` hid that this did not occur.
