FROMLIST: test-ww_mutex: Allow test to be run (and re-run) from userland

In cases where the ww_mutex test was occasionally tripping on
hard to find issues, leaving qemu in a reboot loop was my best
way to reproduce problems. These reboots however wasted time
when I just wanted to run the test-ww_mutex logic.

So tweak the test-ww_mutex test so that it can be re-triggered
via a sysfs file, so the test can be run repeatedly without
doing module loads or restarting.

To use, run as root:
  echo 1 > /sys/kernel/test_ww_mutex/run_tests

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: kernel-team@android.com
Change-Id: Icf3bec7f61bfe104dc62b8d29f84dd4e754d4cb1
Signed-off-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/lkml/20241115235919.1029490-1-jstultz@google.com/
Bug: 306081722