selftests/nolibc: rename Makefile

selftests/nolibc: rename Makefile

The nolibc tests are not real kselftests, they work differently and
provide a different interface. Users trying to use them like real
selftests may be confused and the tests are not executed by CI systems.

To make space for an integration with the kselftest framework, move the
custom tests out of the way.
The custom tests are still useful to keep as they provide functionality
not provided by kselftests.

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250620-nolibc-selftests-v1-3-f6b2ce7c5071@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>