Update testing structure for dbt Labs testing support - postgres

[emmyoop]

Resolves #131

This only adds support for testing with postgres. Postgres is run inside the GitHub runner and I do not need to worry about getting credentials for the various warehouses set up this way.

To add additional adapter testing support in the future, you will just need to modify

• .github/workflows/ci.yml to pass through the env vars
• modify profiles.yml to meet the desire var format (example in dbt-package-testing)
• supported_adapters.env to list the adapters you want to test
• tox.ini to add the commands for the adapter tests

I have also left all CircleCi testing in place for now so you can verify that the new testing format is equivalent. If you choose, you can remove CircleCI testing completely once all adapters are setup with the new tox method.

Why Do We Need These Changes

As explained in #131, by setting up tox as a standard entry-point, the Core development team at dbt Labs can include this package's integration tests (along with other popular dbt packages) as part of our continuous development & delivery of dbt.

[clausherther]

FYI, closing this since we'll be stopping support for this package by the end of year.