selftests: harness: Add kselftest harness selftest

Add a selftest for the kselftest harness itself so any changes can be
validated.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250505-nolibc-kselftest-harness-v4-1-ee4dd5257135@linutronix.de


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