New framework for testing adapters

[jtcohen6]

Contributions

• I have read the contribution docs, and understand what's expected of me.

Link to the page on docs.getdbt.com requiring updates

• These updates will be most relevant for folks building/maintaining external adapters: https://docs.getdbt.com/docs/contributing/building-a-new-adapter#testing-your-new-adapter
• These docs will also be relevant for contributors to dbt-core and adapter plugins. The right place for that information, at a high level of technical detail, is probably in CONTRIBUTING.md (or a page linked from there): Update product docs and dbt Core contributing guidelines with triage notes #1057

What part(s) of the page would you like to see updated?

The section on "Testing your new adapter" is a stub. We should expand it significantly, and/or link to a separate page about dbt functional testing.

Additional information

The work here is still in progress:

• We're still iterating, and haven't yet locked down the exact classes/methods in the adapter interface that each plugin will be required to inherit/reimplement: [CT-281] Make obsolete the test compare tables code now in core/dbt/tests/tables.py dbt-core#4778
• We're still figuring out how we want to package the adapter testing framework + test cases: [CT-313] Package new adapters tests dbt-core#4812

We plan for this to be ready for beta-testing, in line with the upgrade to v1.1.