[Meta] Component testing helpers M1

[tkajtoch]

Summary

We want to provide more guidance on how to test our components and assert the values they display.

Goals

• Publish docs on how to test correct integration with EUI components and what selectors to use
• Publish a test helpers library compatible with major testing frameworks, especially those used in Kibana - jest, cypress, and selenium.
• Integrate the library with Kibana to measure how helpful it really is
• Adjust components if needed to ensure all have static selectors to use reliably no matter what DOM changes we may make in the future

Scope

This milestone is meant to be a research phase and should be scoped to up to two weeks of work.

We should focus on the most used and complex components first to measure if testing helpers actually decrease the time spent on maintaining and upgrading EUI in Kibana.

Definition of done

• Component testing documentation is published to the new docs site
• EUI testing helpers library is published to NPM
• Selected Kibana UI tests are updated to use the new testing library
• A plan forward is prepared and shared with the team

### Tasks
- [ ] https://github.com/elastic/eui/issues/6837