selftests/timens: Make run_tests() functions static

These functions are never used outside their defining compilation unit and
can be made static.

Link: https://lore.kernel.org/r/20250502-selftests-timens-fixes-v1-2-fb517c76f04d@linutronix.de


Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>