Decouple tests from external TestCase dependency

[pierlon]

Summary

Adds a local TestCase class that each test file will now extend from to decouple them from the external Yoast\WPTestUtils\WPIntegration\TestCase class.

Props to @schlessera for recommending it (#6525 (comment)).

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for 48bdf3f are ready 🛎️!

• Download development build
• Download production build

[westonruter]

I suppose you'll cherry-pick this onto a 2.1 branch PR as well?

[pierlon]

I suppose you'll cherry-pick this onto a 2.1 branch PR as well?

Yep will do.