Improve test coverage in callbacks/broker.py

[DominicOram]

Currently broker.py has ~ 44% test coverage. We should improve this to bring it in line with the rest of the project.

[callumforrester]

This is a good idea but should not up coverage for its own sake: Tests should represent use cases.

Acceptance Criteria:

• Tests written against use cases
• Module fully covered

[tacaswell]

The low coverage in that module is because the code both requires significant vertical integration (needs a set up and fed databroker) and requires writing files.

I think adding tests for these is likely to be far more effort than it is worth for (I believe) rarely used consumers.

[callumforrester]

Can it not be written more unit-test-style without the vertical integration? I.e. with mocks etc.

[tacaswell]

You would have to mock out at least DataBroker.fill_events which if you feed it a real document stream is re-implementing a good chunk of the complexity in databroker and I suspect it would be easier to setup the vertical integration on CI!

I'm relatively wary of mocking out complex interfaces. We had a lot of issues early on when we had a "mock ophyd" in bluesky, all the tests would pass....and then we would discover all of the differences between our mock ophyd and real ophyd on the floor!