We recently saw issues where a user could not upgrade Coder when using a custom scheme in the DB.

• migration fails with relation "public.template_versions" does not exist #12619
• DB schema-hardcoding in migration prevents updating to >2.9.0 #12631

To prevent this from happening in the future, we should add tests.

          @matifali we should make tests preventing this from happening in the future.

Originally posted by @kylecarbs in #12631 (comment)