tests: make filemonitor test more robust to event ordering

The ordering of events that are emitted during the rmdir
test have changed with kernel >= 5.3. Semantically both
new & old orderings are correct, so we must be able to
cope with either.

To cope with this, when we see an unexpected event, we
push it back onto the queue and look and the subsequent
event to see if that matches instead.

Tested-by: Peter Xu <peterx@redhat.com>
Tested-by: Wei Yang <richardw.yang@linux.intel.com>
Tested-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>