selftests/timers/nanosleep: Add tests for return of remaining time

selftests/timers/nanosleep: Add tests for return of remaining time

If interrupted by a signal clock_nanosleep() returns the remaining time
into the structure pointed to by the rmtp parameter. So far this
functionality was not tested by the timer selftests.

Extend the nanosleep selftest to cover this feature.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251106-nanosleep-rtmp-selftest-v1-1-f9212fb295fe@linutronix.de