selftests/nolibc: add a new rmdir() test case

A new rmdir_blah test case is added to remove a non-existing /blah,
which expects failure with ENOENT errno.

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>