selftests/nolibc: also test libc-test through regular selftest framework

selftests/nolibc: also test libc-test through regular selftest framework

Hook up libc-test to the regular selftest build to make sure
nolibc-test.c stays compatible with a normal libc.

As the pattern rule from lib.mk does not handle compiling a target from
a differently named source file, add an explicit rule definition.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260106-nolibc-selftests-v1-3-f82101c2c505@weissschuh.net