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