pipeline: Call gst_task_cleanup_all() before checking reference counts after shutdown

We have to ensure that all background threads from thread pools are shut
down, or otherwise they might not have had a chance yet to drop their
last reference to the pipeline and then the assertion for a reference
count of 1 on the pipeline fails.
6 jobs for pipeline-deterministic-clocks in 28 minutes and 36 seconds (queued for 2 seconds)