VSX-PCTS tests the errno returned from an unlink() call on a directory.
Although Linux's setting of errno = EISDIR is *informative*, the Single
Unix Specification requires errno to be set to EPERM for implementations
that do not support unlink() operating on a directory.  See SUS for more
details.  A simple test-case is set up below...

#include <stdio.h>
#include <errno.h>

#define DIRNAME "./testdir"

main()
{
        if (mkdir(DIRNAME, 0755) < 0) {
                perror(DIRNAME);
                exit(1);
        }
        if (unlink(DIRNAME) < 0 && errno == EPERM) {
                exit(0);
        }
        perror("unlink");
        fprintf(stderr, "Expected errno %d, received %d\n", EPERM, errno);
        exit(1);
}