Use a containerized test suite to test more backends

[jeancochrane]

Currently, Postgres is the only backend that is tested in the test suite for this plugin. While this plugin should theoretically work with any database that is supported by SQLAlchemy, it would be nice to have verification that this is true, both to be able to officially support multiple backends and to protect against future changes that might break support for other common databases.

Containerizing the test suite would allow us to test against multiple database backends. At minimum, we should support Postgres, SQLite, and MySQL.