Testing Basics: Add an additional resource

[wishyrater]

This Fireship video gives a quick but useful high-level explanation of Test Driven Development and should be added.

Because

I struggled a lot to understand some of the concepts and terms that were being thrown around. What is the difference between a unit test and an integration test? When would I need to use Setup and Teardown? This video really solidified my knowledge by:

• Giving a high-level explanation of TDD. Basically explaining it in one sentence, which gave me an a-ha moment.
• Explaining the different levels of testing, including unit testing, integration testing and E2E testing.
• Provided useful examples of testing.
• Provided a specific example of when you would need to use Setup & Teardown.
• Provided some useful tips & tricks when using Jest.

This PR

• Added a Fireship video on TDD to the additional resources-section.

Issue

Closes #XXXXX

Additional Information

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