Improvements to tests

[peytondmurray]

This PR introduces a number of changes targeting dev experience:

• Make all tests target databases named test; postgres used to target postgres_db.
• Use ODBC Driver 18 everywhere
• Create containers/ directory; this contains a docker-compose.yml that gets postgres (and now mssql 2022) up and running with a simple docker compose up --build, to make local testing easier
• Removed old pyodbc pytest setup code that fiddles with sys.path. This makes things more complicated when running tests, and I've had no issue running with --import-mode=importlib
• Added pytest setup code that checks which databases are available, and introduces some new pytest marks to facilitate skipping tests if a database service is not up and running