selftests: harness: Add teardown callback to test metadata

To get rid of setjmp()/longjmp(), the teardown logic needs to be usable
from __bail(). Introduce a new callback for it.

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-v4-10-ee4dd5257135@linutronix.de


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