A11y: Improve test runner recipe

Closes N/A

What I did

Also reuse the a11y addon's custom element selector on the test runner, if the user has set it.

Checklist for Contributors

Testing

Manual testing

Did not manual test, but should be simple enough to create a story with a custom element selector and run it in the test runner.

🦋 Canary release

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_core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>_

Greptile Summary

This change enhances the accessibility testing configuration in the Storybook test runner by allowing custom element selectors in story parameters.

Added support for custom element selectors in docs/_snippets/test-runner-a11y-configure.md
Improved flexibility for targeting specific elements in accessibility checks
Fallback to default '#storybook-root' selector if no custom selector is provided
Aligns test runner behavior with the a11y addon's custom element selector feature

storybookjs / storybook