tick: ensure timers are behaving consistently of rewind

[martintmk]

Delivering a consistent experience between the real world and the testing world is super important. If the tests behave in one way, but the real system behaves in a different way, we will be inviting developers to create broken code.

So I think consistency is super important.

Now, changing the fake clock to move the time backwards seems like a "can come later" concern. You could constrain it such that the fake clock can only move forward for now, which would simplify some of the edge cases in the implementation.

Originally posted by @geeknoid in #106 (comment)