Decide how to handle tests and linting in core plugins

[kimjoar]

Extension of #12467

We need to decide how to test and lint core plugins. I think we should just have a single test and linting setup for the entire Kibana repo, instead of having separate configs. Then external plugins can do whatever they want for testing.

However, for integration testing likely need something that external plugins can hook into. See #12699 for the integration test issue.

If we decide on going this route, I think this likely works out of the box with the current Jest setup, we just need to make sure.

[epixa]

I'm going to close this for now. It's likely we will need to do this, but we can create an issue for this if/when we do that has more details. I'm not keen on keeping new platform issues around that aren't concrete tasks, at least for now.