Add high-level overview of testing for PIXL modules

[milanmlft]

Definition of Done / Acceptance Criteria

Fleshed out details of the Test sections in the PIXL module READMEs.
Or, alternatively, have a docs/testing.md file that gives an overview of how tests are run.

Testing

No response

Documentation

Instead of just providing the command(s) to run the tests, add a bit more detail about

• What is being tested
• Which services are spun up to enable the tests
• Any input files required
• Any output files generated

Dependencies

No response

Details and Comments

No response