test wfcqueue: add tests for queue state return value

with e.g. ./test_urcu_wfcq 2 2 10 -w

we can confirm that we see as many "enqueue to empty queue" as we see
"splice from non-empty queue", which confirms that the queue state
returned by enqueue is indeed sampled atomically with enqueue.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>