harness: fix read into PROT_WRITE mmap test
    
    This test has been broken forever.  Fix it up to perform an
    aio_read using the result of a regular read as the expected
    return code.
    
    Reported-by: Guillem Jover <guillem@hadrons.org>
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>