Add comprehensive test suite verifying that magic (referenced) SERVICE_URL_ and
SERVICE_FQDN_ variables don't overwrite values set by direct template declarations
or updateCompose(). Tests cover the fix for GitHub issue #8912 where generic
SERVICE_URL and SERVICE_FQDN variables remained stale after changing a service
domain in the UI. These tests ensure the transition from updateOrCreate() to
firstOrCreate() in the magic variables section works correctly.
