harness: add test for allocating aio-max-nr ioctxs
    
    Ensure that a user is able to allocate aio-max-nr events, even
    if they are spread out 1 to an io_context.  This test code is
    based on a reproducer posted by Mauricio Faria de Oliveira
    <mauricfo@linux.vnet.ibm.com>.
    
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>