selftests: ntsync: Add a stress test for contended waits.

selftests: ntsync: Add a stress test for contended waits.

Test a more realistic usage pattern, and one with heavy contention, in order to
actually exercise ntsync's internal synchronization.

This test has several threads in a tight loop acquiring a mutex, modifying some
shared data, and then releasing the mutex. At the end we check if the data is
consistent.

Signed-off-by: Elizabeth Figura <zfigura@codeweavers.com>
Link: https://lore.kernel.org/r/20241213193511.457338-28-zfigura@codeweavers.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>