Convert tox tests to central workflow

[emmyoop]

Follow-up to #919

Problem

dbt-utils maintaining it's own testing within the workflow. This required a high understanding of how to parse files and run the tests.

Solution

Start using a centralized workflow to ensure testing is done in a consistent way.

Moving to use https://github.com/dbt-labs/dbt-package-testing/blob/main/.github/workflows/ci.yml

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the README.md (if applicable)

[dbeatty10]

Love simplification!