test-ww_mutex: Move work to its own UNBOUND workqueue

test-ww_mutex: Move work to its own UNBOUND workqueue

The test-ww_mutex test already allocates its own workqueue
so be sure to use it for the mtx.work and abba.work rather
then the default system workqueue.

This resolves numerous messages of the sort:
"workqueue: test_abba_work hogged CPU... consider switching to WQ_UNBOUND"
"workqueue: test_mutex_work hogged CPU... consider switching to WQ_UNBOUND"

Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251205013515.759030-3-jstultz@google.com