Update tests, data and code for recent releases
    
    I've been kinda slacking on updating the tests and test data for
    new stable releases, which I'm supposed to do...every time one
    comes out. This adds 28, 29, 30 and 31 all at once. It also:
    
    * Bumps the stupid 'archive cutoff' from < 26 to < 30
    * Adjusts tests for that
    * Drops a bunch of hacks from test_compose_urls (yay)
    * Adds a couple of new subvariants to the constants
    * Adds aarch64 to the constants (it became primary)
    * Rejects EPEL composes as unsupported
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>