selftests/nolibc: introduce condition to run tests only on nolibc

Some tests only make sense on nolibc. To avoid gaps in the test numbers
do to inline "#ifdef NOLIBC", add a condition to formally skip these
tests.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>