Document how to write/run tests for external plugins

[hamishwillee]

Currently running tests on external plugins is broken. The docs have been updated to some extent, but can't be tested (and could be wrong because of that).

Action to update docs on how to do this, once the functionality is fixed:

• [ ] Information on how to run integration tests for external plugins
• [ ] Information on how to run unit tests for external plugins

Note, this follows on from #95