backend: make the walk_limited test not dependend on its output order
    
    This test fails in Koji with the following error
    
        Full diff:
        - [('/tmp/copr-test-walkveaoebpg', ['user2', 'user1'], [])]
        ?                                          ---------
        + [('/tmp/copr-test-walkveaoebpg', ['user1', 'user2'], [])]
        ?
    
    so the `walk_limited` function doesn't guarantee the order of resulted
    directories. Which IMHO isn't a problem. Instead, we should have its
    test not dependent on output order.