openat-tests: ensure unlinkat behaves like rmdir * tests/test-rmdir.c (main): Factor guts... * tests/test-rmdir.h (test_rmdir_func): ...into new file. * modules/rmdir-tests (Files): Ship new file. * modules/openat-tests: New test. * tests/test-unlinkat.c: Likewise. Signed-off-by: Eric Blake <ebb9@byu.net>