Update unit testing guide with code coverage

[dijidiji]

Description

Added instructions on generating code coverage to the guide, corresponding to thoth-tech/splashkit-core#116

Type of change

• Documentation (update or new)

How Has This Been Tested?

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 from ... on the Pull Request

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