runtime: eliminate arbitrary timeout in TestCgoLockOSThreadExit
This test previously failed if running a new pthread took longer than
a hard-coded 100ms. On some slow or heavily-loaded builders, that
scheduling latency is too short.
Since the point of this test is to verify that the background thread
is not reused after it terminates (see #20395), the arbitrary time
limit does not seem helpful: if the background thread fails to
terminate the test will time out on its own, and if the main goroutine
is scheduled on the background thread the test will fail regardless of
how long it takes.