Add tests for Kernel.sleep calling the Fiber scheduler hook

[herwinw]

This is one of the pending issues for Ruby 3.0 (#823). This one has @aardvark179 in front of it, so I'm adding the same disclaimer as I did in #1064: I'm not trying to step on anybody's toes, but I was implementing this logic in Natalie, so I kind of needed these tests anyway.

The tests are pretty barebones checks to see if we call the scheduler with the correct arguments. A generic fiber scheduler has been added that logs the events, this one can be used in similar scheduler hooks tests as welll. If there is need for an integration test kind of spec: I've written one for Natalie, it can be found at https://github.com/natalie-lang/natalie/pull/1287 (with some weird code parts to work around some unrelated bugs), which can pretty much be copy-pasted.