Allow page tests to validate the rendered HTML with axe-core

Page tests (PageTest) render templates and wiki pages to HTML, but nothing checks that HTML for accessibility. Problems such as an icon-only button without an accessible name (see XWIKI-25161) are only caught by the Docker functional tests, and only when the test happens to display the faulty UI, which is often not the case for modals (see XWIKI-22260).

Vue component tests already have an axe-core assertion for this: assertAxe, from xwiki-platform-node/src/main/node/core/test/accessibility, documented at https://www.xwiki.org/xwiki/bin/view/documentation/xs/dev/front-end/test/test-vue-component-accessibility/

Proposal

Provide a similar helper for page tests, so that a test can run axe-core on the HTML it renders and fail on violations, the same way assertAxe does for Vue components.