Implement adapter testing framework

[jtcohen6]

Très bon work on getting this up + running + onto pypi, Fabrice !

We've just released a new testing framework, pytest-dbt-adapter:

• https://github.com/fishtown-analytics/dbt-adapter-tests
• https://pypi.org/project/pytest-dbt-adapter/

This replaces the older dbt-integration-tests framework, whcih was was a neat experiment with behave that served us well for the past year. We found it wasn't a sustainable way to support the matrix of functionality across databases. With the flexibility of the pytest-based approach, each adapter can declare its supported features by overriding or disabling the predefined test sequences.

Running tests with pytest-dbt-adapter would look like adding a file dremio.dbtspec and executing pytest path/to/dremio.dbspec. The repo linked above contains sample specs for Postgres, Spark, and Presto. You can also see how we've hooked up the tests to run via tox + CircleCI in the dbt-spark and dbt-presto repos.

We'd like to support you in adopting this testing framework, whether that's by writing the code, answering questions, or making needed adjustments to the existing tests. Let me know!