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

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.

This has been particularly valuable to stressing and finding
issues with the proxy-exec series.

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

Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251205013515.759030-4-jstultz@google.com