selftests/timers/posix-timers: Add a test for exact allocation mode

The exact timer ID allocation mode is used by CRIU to restore timers with a
given ID. Add a test case for it.

It's skipped on older kernels when the prctl() fails.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/8734fl2tkx.ffs@tglx