chore: add testing to docs

docs/developer-guide.md

@@ -15,3 +15,9 @@ If you want to see more HTML output, add `DEBUG_PRINT_LIMIT=1000000` (or any lar
 If you want to access the testing playground, add `screen.logTestingPlaygroundURL()` to your test (although if the URL is too long, the playground will be blank).
 
 Front-end unit tests include snapshots. Work that changes the DOM will result in a diff from the last accepted snapshot and cause related tests to fail. You can update the snapshots and review / accept the diff with `yarn test -u`.
+
+### Backend unit tests with Pytest
+
+In the terminal navigate to the desired directory for testing, or the root to run all tests.
+
+Run `pytest` to run all tests or `pytest -v` to get verbose output.