How to run Rails 7 tests or RSpec without separate test database(s)

I have Rails 7.0 applications that query databases of other business applications. The application has to cope with data as it exists in the vendor’s database. We have no control over the schemas, and have read-only access to these databases. Rake would not be able to drop objects or seed the database even if I wanted to. Application uses several such databases. One DB connection is established in the usual way by Rails. Additional connections are established like: