backend: make the walk_limited test not dependend on its output order, pt2
    
    Complements PR#1798
    
    The Koji error looks like this
    
        >   assert root == os.path.join(results, "user1/foo/fedora-rawhide-x86_64")
        ...
        E     - /tmp/copr-test-walkpdbk53nt/user1/foo/fedora-rawhide-x86_64
        E     + /tmp/copr-test-walkpdbk53nt/user1/foo/srpm-builds
    
    I am attempting to sort the output so it is at least somehow
    deterministic. And also fixing the same issue as in PR#1798 except a
    few lines lower.
    
        E   AssertionError: assert ['111', '222'] == ['222', '111']
        E     At index 0 diff: '111' != '222'
        E     Full diff:
        E     - ['222', '111']
        E     + ['111', '222']