Update testing structure for dbt Labs testing support - postgres

[emmyoop]

Resolves: #1700

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

Why Do We Need These Changes

As explained in #1700, 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.