This test called `yield` 80,000 times, which is nothing on a system with
little load, but murder on a CI system. macOS' scheduler in particular
doesn't seem to deal with this very well. The `yield` calls also weren't
even necessarily doing what they were meant to: if the optimizer could
figure out that it doesn't clobber some memory, then it could happily
reorder around the `yield`s anyway!

The test has been simplified and made to work better, and the number of
yields have been reduced. The number of overall iterations has also been
reduced, because with the `yield` calls making races very likely, we
don't really need to run too many iterations to be confident that the
implementation is race-free.