test-unlinkat: enhance test, to expose Solaris 9 bug

test-unlinkat: enhance test, to expose Solaris 9 bug

Share the unlink tests with unlinkat.

* tests/test-unlink.c (main): Factor guts...
* tests/test-unlink.h (test_rmdir_func): ...into new file.
* tests/test-rmdir.h (test_rmdir_func): Add parameter.
* tests/test-rmdir.c (main): Adjust caller.
* tests/test-unlinkat.c (main): Likewise.  Add unlink tests.
(unlinker): New helper function.
(rmdirat): Enhance check.
* modules/rmdir-tests (Depends-on): Add stdbool.
* modules/unlink-tests (Depends-on): Likewise.
(Files): Add test-unlink.h.
* modules/openat-tests (Files): Likewise.
(Depends-on): Add unlinkdir.

Signed-off-by: Eric Blake <ebb9@byu.net>