Disable updates-testing during upgrade_preinstall
    
    This issue appeared when we started testing Rawhide updates, but
    I only noticed it today. When testing Rawhide updates after
    Branch point, the upgrade tests upgrade from Branched to Rawhide.
    On Branched, updates-testing is enabled by default. We only
    disable it when we reach `upgrade_run`, but by that point we've
    already done a `dnf -y update` in `upgrade_preinstall` and
    potentially installed other packages in steps between
    `upgrade_preinstall` and `upgrade_run`. That can cause problems,
    like today all FreeIPA upgrade tests on Rawhide are failing
    because there's a newer freeipa in updates-testing for F37 than
    is in the current Rawhide compose.
    
    Solve this by disabling updates-testing before we do the update
    in `upgrade_preinstall`. To avoid excessive code duplication,
    factor out the repo disabling code.
    
    We'll do this twice on upgrade tests now, but it shouldn't be a
    problem.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>