Package testing documentation

[trudeaua21]

Changes

• Adds documentation for package testing setup tasks, caveats, and configuration options which will be relevant once the package testing PR has been merged into the Unity test runner.

Checklist

• [x] Verify build works
• [x] Verify Links between markdown sections work (wasn't able to test without building)
• [x] Read the contribution guide and accept the code of conduct
• [x] Readme (updated or not needed)
• [x] Tests (added, updated or not needed)

NOTE: I'm blocked from completing the two remaining checklist items by #397 . Since the contributing guidelines say I should ensure there are no build errors before submitting the PR, I'll leave it in draft status until I can figure out a solution or workaround to my issue.

That said, this PR has all of the changes I planned to make - I'm only leaving it a draft for now as a formality.

Issues have been resolved!

[github-actions[bot]]

[trudeaua21]

Resolved my build issues - this should be good for review now!