v.io/x/lib/nsync: Once again allow for slow machines when running timing test
v.io/x/lib/nsync: Once again allow for slow machines when running timing test

cv_timeout_stress_test occasionally runs slower that 75% of expected speed on our test machines.
	cv_timeout_stress_test.go:113: expected more than 67500 timeouts, got 64803
Changing the expactation to 50% to avoid false alarms.

Change-Id: Ifb7fd7f5d6cd5432b8ef6bbfab75771876dc7af8

vanadium/go.lib