Speed up version migration specs (#3994)

Avoid running the slow `load_from_yaml` step when testing parts of the version migrations that don't rely on having the proper code lists loaded. On my machine, this improves the speed of the version migration specs from ~7 minutes to ~2 minutes. In CI, these specs take much, much longer (something like an hour by themselves), so hopefully there will be a proportional boost to the CI build speed.