selftests/nolibc: mark test helpers as potentially unused

When warning about unused functions these would be reported by we want
to keep them for future use.

Suggested-by: Zhangjin Wu <falcon@tinylab.org>
Link: https://lore.kernel.org/lkml/20230731064826.16584-1-falcon@tinylab.org/


Suggested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/lkml/20230731224929.GA18296@1wt.eu/


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>