[CIVIS-1630] DOC document best practices for testing

[jclairelopez]

This PR is for adding documentation for best practices when testing, in reference to JIRA ticket 1630. I used an internal civis function and the python mocking library in line with best practices.

Thanks for the review, and please let me know if you see anything that you'd like done differently!

Thanks Jackson!

Closes #306

• [x] (For Civis employees only) Reference to a relevant ticket in the pull request title
• [x] Changelog entry added to CHANGELOG.md at the repo's root level
• [x] Description of change in the pull request description
• [x] If applicable, unit tests have been added and/or updated
• [x] The CircleCI builds have all passed