Include a mock store in tests

Tests might need to run their own SQL. To do this, some modules implement a mock store that runs against an in-memory database - see https://github.com/vector-im/tchap-red-list/blob/ce6fbbbcbf3b5a9aa5504ce362ded2203a6f5b5e/tests/__init__.py#L27-L81 for an example. The template should probably include this in the test suite it generates.

Start with the referenced tests/__init__.py example, especially lines 27-81, and compare it with the test suite generated by the template. Add equivalent mock-store coverage for running SQL against an in-memory database, then verify that newly generated module tests include it.

Written by the indexing model from the issue text.