v.io/x/lib/nsync: fix flaky deadline test in condition variables

The tests TestCVDeadline and TestCVCancel use time, since they are
testing a time-dependent feature of condition variables.  They
sometimes fail because the scheduler fails to run threads promptly.

This change count how often they miss their deadlines, and fails
the test only if they miss sufficiently several times.

Change-Id: Id7d6032f1e569abe24a6a67f699516c727be511c
1 file changed