Add docs for testing module

[znichollscr]

• (N/A) Closes # (insert issue number)
• Executed pre-commit run --all-files with no errors
• (N/A) The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[znichollscr]

@hgrecco or @andrewgsavage are you able to approve the workflow and review this? I think the change is pretty trivial and means that there are now entries for the testing functions in the docs.

Current main

This MR

[codspeed-hq]

CodSpeed Performance Report

Merging #2070 will not alter performance

_{Comparing znichollscr:testing-docs (2ca31aa) with master (861953e)}

Summary

✅ 437 untouched benchmarks