[META] Simplify / streamline codebase to allow accessibility tests to be extended for contrib themes

It would be really great if there was a simple process to extend core's tests for use in a contrib module to ensure that all of the accessibility improvements are not accidentally broken by the contrib module.

Initial idea:

• Refactor / streamline / simplify the tests around accessibility so that they have a hierarchical structure:
  • Base tests for system output.
  • Each bundled theme would extend these to include any additional accessibility adjustments necessary for the theme, e.g. DOM-level adjustments where the output adds extra DIVs, etc.
• Clearly document how a contrib theme would go about leveraging the core tests.
• Later on.. create a generator in DrupalConsole to create the necessary test files for a given subtheme.