Accessibility tests

[nwmac]

Summary

Fixes #

Occurred changes and/or fixed issues

This PR is a re-work and update to #9457.

Technical notes summary

This PR adds a new e2e test suite a11y-tests which run a set of e2e tests with support for checking accessibility using the cypress-axe plugin.

This is done as a separate test suite, so that we get an easy-to-run suite that produces a single audit report, rather than the report being spread across multiple suites.

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
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• The PR has been self reviewed
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• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes