selftests/timers/posix_timers: Test delivery of signals across threads

Test that POSIX timers using CLOCK_PROCESS_CPUTIME_ID eventually deliver
a signal to all running threads.  This effectively tests that the kernel
doesn't prefer any one thread (or subset of threads) for signal delivery.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230316123028.2890338-2-elver@google.com