improve future::poll calibration loop

improve future::poll calibration loop

The calibration loop I've recently added to the libstdc++
future/members/poll.cc tests could still select iteration counts that
might yield zero-time measurements for the wait_for when ready loop.

Waiting for a future that has already had a value set is presumably
uniformly faster than a zero-timed wait for a result, so I've changed
the calibration loop to use the former.

We might still be unlucky and get nonzero from the initial loop, so
that the calibration is skipped altogether, but then get zero from the
later when-ready loop.  I'm not dealing with this case in this patch.

for  libstdc++-v3/ChangeLog

	* testsuite/30_threads/future/members/poll.cc: Use faster
	after-ready call in the calibration loop.