🛠️ Unit Testing: Establish Robust Unit Testing

[Ipstenu]

Right now we're not testing everything we could/should be (for example Mocking installations and upgrades)

What we need is:

• A list of things we should be testing
• A list of existing tests on .org we can fork

[kasparsd]

Noting that #383 proposes moving all testing implementation to wp-env and allows any WP+PHP combo (used by the CI matrix as well). I have a pull request ready and am waiting for #382 to get merged since it is a follow-up to that cleanup and documentation updates.