Write better database unit tests

The current database is unreliable (tables are not always correctly populated). To better identify and fix bugs, we need more, better unit tests and ensure that the db passes them. This will be especially important after we implement new [config](https://github.com/facebookresearch/aepsych/issues/33#issue-1238780434) and [data](https://github.com/facebookresearch/aepsych/issues/34#issue-1238791216) tables.