[libstdc++-v3] [testsuite] improve future/*/poll.cc calibration

30_threads/future/members/poll.cc has calibration code that, on
systems with very low clock resolution, may spuriously fail to run.
Even when it does run, low resolution and reasonable
timeouts limit severely the viability of increasing the loop counts so
as to reduce measurement noise, so we end up with very noisy results.

On various vxworks targets, high iteration count (low-noise)
measurements confirmed that some of the operations that we expected to
be up to 100x slower than the fastest ones can run a little slower
than that and, with significant noise, may seem to be even slower,
comparatively.

Bump the factors up to 200x, so that we have plenty of margin over
measured results.


for  libstdc++-v3/ChangeLog

	* testsuite/30_threads/future/members/poll.cc: Factor out
	calibration, and run it unconditionally.  Lower its
	strictness.  Bump wait_until_*'s slowness factor.