Write documention that describes how to run and add tests in schema repo about bods-dev-handbook

[kd-ods]

To add some detail. The documentation should explain:

• What the test setup is in the repo. (e.g. where the tests reside ... /test directory)
• What the tests do or look for.
• Whether and when the tests are run automatically.
• How they can be run locally (e.g. for development)
• Whether they can be run in isolation (eg just test_csv.py)
• How to add test data (is 'fixtures' the right term??). Is it important, eg, that test json files in the data/*-statement directories only have a single statement object in them (not an array)? (At the moment, the PLCs files in the entity-statement directories are are arrays.)

(@siwhitehouse @ScatteredInk - anything to add here?)

[ScatteredInk]

Thanks @kd-ods, this looks good.

Whether and when the tests are run automatically.

I think an overview of the CI/build process would be helpful, including the rules on merging - this feels like an assumption at the moment.

We might also benefit from a high-level statement of what should be tested and what is tested, e.g. test all example data, code conventions.

[kd-ods]

This ticket in the main repo can be closed once the dev handbook is updated with test info.