Add testing guidelines to the Wiki

[Heatmanofurioso]

Is your feature request related to a problem? Please describe.
Many developers seem to either not work on testing, or not understand the what, how and why of testing the various features.

Describe the solution you'd like

• Add more value to the wiki.
• Detail some sources to read on testing.
• Working on our guidelines for testing.
• Maybe start to work towards one day enforcing tests on PRs for new features.
• Mention some tests that we already have working on the wiki as examples. Maybe further document these classes and tests to help out new people to better understand how they can make their own tests + fix any tests that they might break in their implementations or understand why the test broke and what are they doing wrong that the test is actually helping prevent them from merging.

Describe alternatives you've considered
None

Additional context
None to give.

[github-actions]

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