Theme integration test suite

## Theme integration test suite

We want a theme gallery (https://github.com/facebook/docusaurus/issues/3522) and share UX between themes.

This should enable to create a single Cypress integration test suite to validate the behavior of all the themes, as long as the test selectors are shared between each theme.

As the classic theme is the most advanced and the "reference" theme, we can use it to get started asap, until other themes catch up in feature parity.

We should think carefully about creating `testId` and aria-roles so that selectors are shared between each tests

Some work has already been done by @fanny here: https://github.com/facebook/docusaurus/pull/3468