tests: avoid coroutine pool test crash

Skip the test_co_queue test case if the coroutine pool is not enabled.
The test case does not work without the pool because it touches memory
belonging to a freed coroutine (on purpose).

Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1463767231-13379-1-git-send-email-stefanha@redhat.com
