selftests/timens: Print TAP headers

The TAP specification requires that the output begins with a header line.
These headers lines are missing in the timens tests.

Print such a line.

Link: https://lore.kernel.org/r/20250502-selftests-timens-fixes-v1-1-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>