comps: Add tests for CompsFilter
    
    All use cases that are actually used by pungi-koji are tested. There is
    missing coverage for
    
     * keeping only items with matching arch
     * not reindenting the file
    
    These aren't currently used and should be removed in the future, but
    there may be other tools depending on the comps_filter executable.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>