Codemod some expiration tests to waitForExpired (#26491)

Continuing my journey to migrate all the Scheduler flush* methods to
async versions of the same helpers.

`unstable_flushExpired` is a rarely used helper that is only meant to be
used to test a very particular implementation detail (update starvation
prevention, or what we sometimes refer to as "expiration").

I've prefixed the new helper with `unstable_`, too, to indicate that our
tests should almost always prefer one of the other patterns instead.