Miri now randomly yields the current thread every now and then, but currently it always yields to the next thread, i.e. it's a round-robin scheduler. It would probably be good to randomize this more and pick some arbitrary unblocked thread whenever we have yielded.

To still provide a fixed schedule for testcases, we could have a flag -Zmiri-fixed-schedule that disables both automatic yielding and that forces things to be round-robin again. All existing tests that use -Zmiri-preemption-rate=0 should then use this new flag instead.