harness: add test for aio poll missed events
    
    Add a regression test for a recently-fixed kernel bug where aio polls
    sometimes didn't complete even if the file is ready.
    
    This is a cleaned-up version of the test which I originally posted at
    https://lore.kernel.org/r/YbMKtAjSJdXNTzOk@sol.localdomain
    
    Signed-off-by: Eric Biggers <ebiggers@google.com>
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>