fedora-common-ostree: Disable updates-testing repos
    
    For released versions of Fedora, the following refs are composed:
    
    ```
    $ ostree remote refs fedora | grep x86 | grep silverblue | grep 39
    fedora:fedora/39/x86_64/silverblue
    fedora:fedora/39/x86_64/testing/silverblue
    fedora:fedora/39/x86_64/updates/silverblue
    ```
    
    The unprefixed ref is composed with the release and updates repos
    enabled. The `updates/silverblue` ref is an alias, likely for
    compatibilty.
    
    The `testing/silverblue` ref is composed with the updates-testing repos
    additionnaly enabled, thus including packages that are still in testing.
    
    For branched versions of Fedora, the following refs are composed:
    
    ```
    $ ostree remote refs fedora | grep x86 | grep silverblue | grep 40
    fedora:fedora/40/x86_64/silverblue
    fedora:fedora/40/x86_64/testing/silverblue
    ```
    
    The updates-testing repos are enabled by default in Fedora only during
    the time between the beginning of the Beta freeze and the beginning of
    the Final freeze.
    
    See: https://docs.fedoraproject.org/en-US/quick-docs/fedora-repositories/#the-updates-testing-repository
    See: https://src.fedoraproject.org/rpms/fedora-repos/commits/f40
    
    During this time, users that install test systems from ISOs of an
    upcoming release will thus be using the non-prefixed ref that only
    includes packages from the stable repos, but will have the
    updates-testing repos enabled on their systems.
    
    Local package layering operations will thus often result in unexpected
    dependency conflicts.
    
    See: https://github.com/fedora-silverblue/issue-tracker/issues/549
    
    Ideally we would have the updates-testing repos enabled only for the
    composes that are built with those repos enabled (`testing/silverblue`).
    But I currently don't know how to figure that out in the Fedora
    infrastructure.
    
    Thus we make sure that the updates-testing repos are always disabled by
    default. Users of the testing refs doing package layering will have to
    enable them manually.
    
    Fixes: https://github.com/fedora-silverblue/issue-tracker/issues/549