chore(lint): add and apply eslint-plugin-testing-library

[the-ult]

What

Add eslint-plugin-testing-library and eslint-plugin-jest-dom and fix the Errors found

Why

The examples provided should be following the standards/best practices of the testing-library.

Checklist

• Ready to Merge

fixes #198

[the-ult]

@timdeschryver
What would be the proper way to fix this?

.../angular-testing-library/apps/example-app/src/app/examples/15-dialog.component.spec.ts
57:28  error  
- Avoid direct Node access. Prefer using the methods from Testing Library  testing-library/no-node-access

  fireEvent.click(document.querySelector('.cdk-overlay-backdrop'));

https://github.com/testing-library/eslint-plugin-testing-library/blob/main/docs/rules/no-node-access.md

[timdeschryver]

@the-ult You can add a disable rule comment where that error occurs.
AFAIK, there isn't an "accessible" way to click on the backdrop.
Besides that, this LGTM 👍

[timdeschryver]

Thanks for the migration @the-ult !

[github-actions]

🎉 This PR is included in version 10.6.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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