harness: improve _wait_for_clock_id_waits performance
By moving the functionality down to the testclock, the implementation no longer needs to poll the waits, but rather wait properly for them to be added.
The performance-hit here would be that by polling the test-clock regularly, you would create contention on the testclock-lock, making code using the testclock (gst_clock_id_wait) fighting for the lock.
Fixes #288 (closed)