chore: add postgres dbt model to dbt tests

[talagluck]

I know there are issues here, but sharing for early review:

• Changes the expected test outcome for the view materialization in the existing dbt test from fail to pass
• Adds a test confirming that dbt will work with an external postgres database. This uses the actual external demo postgres instance that we mention in our docs, so is probably brittle. Ideally, we would spin something up (Docker?) for the purpose of the test.
• Overall, this could also probably use some refactoring, but I think that that will take some pytest magic which may make the tests less legible.

[tychoish]

this all seems good.