Use Jest's fake timers instead of a custom stub

[mcmire]

When rewriting the tests in Jest, I used a custom stub for setTimeout / clearTimeout instead of using Jest's fake timers. This turned out to be easier to use at the time because of the way that we use setTimeout in PollingBlockTracker. While writing those tests, I ended up adding some custom events that proved to be useful, one of which is _waitingForNextIteration. Now that this exists, I believe that this is the key to using Jest's fake timers.

[mcmire]

Could also consider flushing promises instead of using _waitingForNextIteration: https://github.com/MetaMask/controllers/pull/1002/files#diff-ead58d4e73060469003c03b4dce6c74d920a2e2357ce7c0e15c3bc8165aad4abR41