selftests/nolibc: anchor paths in $(srcdir) if possible

It is easier to recognize paths from their well-known location in the
source tree than having to resolve the relative path in ones head.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20231031-nolibc-out-of-tree-v1-2-47c92f73590a@weissschuh.net