Missing tests for rb_io_maybe_wait_*, e.g., rb_io_maybe_wait_readable

[eregon]

I noticed https://github.com/ruby/ruby/blob/master/ext/-test-/wait/wait.c, but the methods defined there like io_maybe_wait_readable don't seem used anywhere.
In fact I couldn't find any test for rb_io_maybe_wait_* functions.

The TruffleRuby team will add some basic specs based on the non-maybe variants of these functions.

It would be useful to add some MRI tests for this, and also test with a scheduler.