Automate test coverage check on pull requests

[Jacob Walls]

We could automate a significant part of checking test coverage on pull requests. Instead of using a cloud service, there's an implementation using diff-cover in a PR.

​PR

​Forum post

[Simon Charette]

I think the last time this was attempted the efforts stalled due the difficulty of combining coverage reports from different suite runs. Some part of the code is only covered by tests run on Postgres or a particular Python version for example so there needs to be a coordinated job that collects all of the .coverage artifacts and ​then combine them otherwise the resulting coverage report will be lacking or improperly reporting that some areas are not covered (e.g. if we only use the SQLite test run and Postgres only changes are introduced).

[Jacob Walls]

(The linked implementation just treats that complexity as outside of MVP and makes this caveat explicit in the posted comment.)