[libstdc++] [testsuite] avoid async.cc loss of precision [PR91486]

When we get to test_pr91486_wait_until(), we're about 10s past the
float_steady_clock epoch.  This is enough for the 1s delta for the
timeout to come out slightly lower when the futex-less wait_until
converts the deadline from float_steady_clock to __clock_t.  So we may
wake up a little too early, and end up looping one extra time to sleep
for e.g. another 954ns until we hit the deadline.

Each iteration calls float_steady_clock::now(), bumping the call_count
that we VERIFY() at the end of the subtest.  Since we expect at most 3
calls, and we're going to have at the very least 3 on futex-less
targets (one in the test proper, one before wait_until_impl to compute
the deadline, and one after wait_until_impl to check whether the
deadline was hit), any such imprecision that causes an extra iteration
will reach 5 and cause the test to fail.

Initializing the epoch in the beginning of the test makes such
spurious fails due to loss of precision far less likely.  I don't
suppose allowing for an extra couple of calls would be desirable.

While at that, I'm annotating unused status variables as such.


for  libstdc++-v3/ChangeLog

	PR libstdc++/91486
	* testsuite/30_threads/async/async.cc
	(test_pr91486_wait_for): Mark status as unused.
	(test_pr91486_wait_until): Likewise.  Initialize epoch later.