Update unit testing guide (original PR#150)

[rory-cd]

Description

This PR is a copy of #150, created by former contributor @dijidiji. The original PR had tests failing, which I have fixed along with some minor updates:

• Prerequisites added back in, and shifted to their own section (these are required whether you use command line or VS Code)
• Renamed "Unit Testing Guide" to "Writing Unit Tests", to make it clear this section only covers writing tests, not background info, building or running.

Original Description (#150)

I've revised and updated the unit testing guide. I've also merged previous testing instructions for sktest with relevant instructions that were part of the unit testing guide, since the build process builds both. As part of this, I've elaborated on the differences between sktest and skunit_tests a bit, and moved the part of the guide that outlined how to write a unit test to a separate page.

This way, someone following the guide is made aware of how to run both test programs, and is informed of how to add tests to either of them (as well as the reasons for doing so).

• This also fixes an issue where resources for tests (such as images) aren't copied, causing tests to fail. This happens because building a specific executable target such as skunit_tests doesn't run the custom targets used for the copy. This could be fixed by altering CMakeLists.txt but updating this guide is simpler.

Type of change

• Documentation (update or new)

How Has This Been Tested?

Made sure the guide looks good in local testing.

Testing Checklist

• Have run npm run format
• Have run npm run build
• Have run npm run dev and/or npm run preview, using:
  • Chrome
  • Safari
  • Firefox

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have requested a review on the planner board

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