test: add a11y tests to showcases

[nmerget]

Proposed changes

closes https://github.com/db-ui/mono/issues/2571

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Refactoring (fix on existing components or architectural decisions)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update (if none of the other choices apply)

Further comments

[github-actions[bot]]

🔭🐙🐈 Test this branch here: https://db-ui.github.io/mono/review/test-a11y-showcases