Allow scheduling tests for multiple Koji tasks together
    
    This is a bit of a pain to shoehorn into the way the code is
    written, and I might clean it up later, but it's a useful
    capability: run update tests on *multiple* tasks together. We
    need it for cases like https://gitlab.gnome.org/GNOME/mutter/-/issues/2918#note_1801378 .
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>