Document setup steps for databases for CI tests

[dbeatty10]

Describe the feature

The CI tests for this dbt package run against the following databases:

• Postgres
• BigQuery
• Redshift
• Snowflake

We want to be able to set these up from scratch and know how to configure them for use in CI. This includes things like necessary permissions, expected settings that differ from the default, and how to accomplish authentication/authorization via environment variables.

So we should create documentation that outlines all of the necessary steps to set these up from scratch.

Who will this benefit?

What kind of use case will this feature be useful for? Please be specific and provide examples, this will help us prioritize properly.

This will directly benefit the maintainers of this package by empowering setup from scratch. It could also indirectly benefit maintainers of other dbt Labs packages for the same reason.

Anything else?

Instigated by:

• Add support for bigquery testing in GitHub CI via tox #203
• Add support for redshift testing in GitHub CI via tox #204
• Try removing Redshift-specific logic for toggling case-sensitive identifiers #208