Decouple kernel tests from XbTestSetup

💯 — I've always been opposed to this.

Found this via https://git.drupalcode.org/project/experience_builder/-/merge_requests/1... btw, where you wrote:

ℹ️ Moved this away from XbTestSetup because this test was burning time working on fixing it.
See #3531679 for follow-up to do the rest of our Kernel tests in this boat. significant DX and velocity boost - before time to execute the 5 test methods is 3 mins 20 seconds, after is 28 sec

For context: this was the pragmatic choice at the time. 😞 The Page content entity type has not yet existed for that long, so for a long time you HAD to install the article node type with the right fields etc in just the right way to get something working to develop against.

In 📌 Refactor(or attempt to) all or most ApiLayoutController*Test classes to also test ContentTemplate entities Active we tried to use ContribStrictConfigSchemaTestTrait but we couldn't because of this issue. So it would be good to add that if we can in this issue