Tox for running CI tests

[dbeatty10]

Describe the feature

Add tox to dbt_utils so that:

• it aligns with the testing infrastructure of other repositories like dbt-core, etc.
• package maintainers can use pytest, dbt unit tests, data tests, something else entirely, or any combo of those options during CI testing

Describe alternatives you've considered

Status quo.

Who will this benefit?

This is intended to empower the maintainers of dbt_utils or anyone else that depends on dbt_utils to have more options for running the test suite(s).