wfstack test: test if number of push to empty vs pop_all match

Do same as wfcqueue: we can test if number of push to empty stack match
the number of pop_all that return non-empty stack.

Can be tested with:
./test_urcu_wfs 5 5 10 -w

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>