test: Add postgres testing

[anticorrelator]

resolves #2783

• Adds a parametrized session fixture, this can be used to parametrize database tests to include either an empty sqlite database session or postgresql database session.
• Refactors data fixtures to be included individually on tests, database fixtures can be populated just with necessary data
• Adapts existing DSL tests to use new database testing setup
• postgres tests can be skipped by passing the --skip-postgres flag to pytest
• in CI, postgres tests only run in linux

[axiomofjoy]

Looks really nice. Let me know if you need help resolving conflicts.

[axiomofjoy]

Think we need to update our developer guide to include instructions on installing postgresql, e.g., via brew.

[axiomofjoy]

nice work! let's remove the nest asyncio test cases and make them into proper unit tests, otherwise, looks good.