Add test for skip links in pytest and cypress

[amazingphilippe]

Summary | Résumé

The pytest makes sure we don't remove the skip link. And runs on every PR

The cypress test makes sure the skip link behaves as it should:

• Be the first focusable item in the document
• Moves focus to the main content when selected

Test instructions | Instructions pour tester la modification

• Tests should pass
• Run cypress skip_links test

[github-actions]

🧪 Review environment

https://p6titwyjch63vt52cc2t62qm5u0ailta.lambda-url.ca-central-1.on.aws/