selftests: harness: Implement test timeouts through pidfd

Make the kselftest harness compatible with nolibc which does not implement
signals by replacing the signal logic with pidfds.
The code also becomes simpler.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250505-nolibc-kselftest-harness-v4-7-ee4dd5257135@linutronix.de


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>