selftests: futex: Add futex compare requeue test

Add testing for futex_cmp_requeue(). The first test just requeues from one
waiter to another one, and wakes it. The second performs both wake and
requeue, and checks the return values to see if the operation woke/requeued
the expected number of waiters.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Link: https://lore.kernel.org/r/20210531165036.41468-3-andrealmeid@collabora.com