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.

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