selftests/timens: Add a test for futex()

Output on success:
 1..2
 ok 1 futex with the 0 clockid
 ok 2 futex with the 1 clockid
 # Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201015160020.293748-2-avagin@gmail.com