Add an architecture for hosts to opt-in to automated testing of plugins

[getsource]

This ticket is to discuss the inclusion of a way to allow hosts to opt-in to automated testing of plugins, in addition to WordPress.

The current stated goal of the test runner is to Run WordPress automated tests on a diverse set of hosting environments to better ensure the quality of the WordPress project.

• Should we extend this to include automated tests from plugins?
• If yes, what would be the best way to allow plugins to list themselves, and for hosts to opt-in?
  Discussion, ideas, and PRs/proof of concept really appreciated.

Created following a discussion on Slack started by @jdevalk.

During the discussion, @jdevalk linked to the WordPress SEO tests as one example of a plugin that has automated tests that could be included.