[libcxx testing] Remove ALLOW_RETRIES from two futures tests

These two tests do not use the "thread sleeps X milliseconds" pattern
that other libcxx tests use, so all we can do in order to remove
ALLOW_RETRIES workaround is remove the assumption that measuring the
"quick" return of `wait()` is possible (it is not). Let the test harness
verify overall that `wait()` does not hang.

As a bonus, have the spin-waiting threads `yield()`, which is what well
behaved code should do.