Fix infinite loop if semaphore test is not run.

When using the --benchmark_filter option, all of the test objects
get created, but not all are run. Previously, if this test didn't run
it would get into an infinite loop waiting for the test to complete.
This change only waits for the test to complete if it was actually
executed.

Change-Id: I5151a0b4b3d5349b978e716ec4a02ebd8b4eae00