Try to fix IdleTime unit test

Try to fix IdleTime unit test

Summary:
IdleTime unit test has been failing randomly (#5996886). The
event base loop calculates an exponentially moving average of loop busy
time in each loop. In the test, the busy time is initialized to 5900 and
each loop runs more than 8000 us. In normal case decaying of the
previous loop busy time will take 6 loops to  move this average to >6000.
But if the test is running on a heavily loaded machine the busy+idle time
a loop is longer than it should be, causing the decaying happen faster.
This diff skips this test if this happens.

Test Plan:
It won't affect anything if running on my devserver. It only
helps on a heavily loaded contbuild host.

Reviewed By: alandau@fb.com

Subscribers: folly-diffs@

FB internal diff: D1788450

Tasks: 5996886

Signature: t1:1788450:1421460774:1ec575c50f881e10b5a0208717fe68164f0d0f57