calibrate intervals to avoid zero in futures poll test

We get occasional failures of 30_threads/future/members/poll.cc
on some platforms whose high resolution clock doesn't have such a high
resolution; wait_for_0 ends up as 0, and then some asserts fail as
intervals measured as longer than zero are tested for less than
several times zero.

This patch adds some calibration in the iteration count to set a
measurable base time interval with some additional margin.


for  libstdc++-v3/ChangeLog

	* testsuite/30_threads/future/members/poll.cc: Calibrate
	iteration count.