Expand tests

[DamianZaremba]

We have the start of some unit test and integration tests (using mock database data), these should be expanded over time.

An example issue which would be easy to pick up in testing:

• Empty prev_user #16
• build_wp_edit - set prev_user if previous_revision is complete #17

This has value in the medium/long term so we don't break the somewhat fragile data loading when things change.

[DamianZaremba]

The replica test logic was re-worked in #28 so should now be safe/easier to expand.

Given the tests are a bit slow and can easily be run in parallel, making that happen is probably the next step (drop the default db include, then run each class in parallel).

Once there is test coverage for all training functions, a couple of "complete" tests checking WPEdit should be made, as we modify the data in that layer before feeding into TrainingData

[DamianZaremba]

#31