Use Jest.useFakeTimers in IO debounce/throttle tests

[andywhite37]

I noticed the IO debounce/throttle tests fail sometimes, seemingly from timing issues. In the Relude_Timer, Relude_Debounce, and Relude_Throttle tests, I tried the Jest.useFakeTimers/Jest.advanceTimerBy (or whatever) API and it seemed to work really nicely, so we should switch these IO tests to use that too.

[andywhite37]

Note: I found that when using Jest.useFakeTimers, the Js.Date values are not affected by the spoofed timer changes (which makes sense in hindsight), so we'll have to figure out a different way to run these tests with more deterministic timing, while still testing the timing aspect of them.

[andywhite37]

Here's an example of using the fake timers in the Relude_Debounce test

https://github.com/reazen/relude/blob/master/__tests__/Relude_Debounce_test.re