Add End-To-End (E2E) tests for BuddyPress

[espellcaste]

We recently started a conversation to add a checklist of steps to test before releasing a new BuddyPress version. This process is usually manual. See ​https://github.com/buddypress/bp-documentation/issues/288

I'd like to ammend this convo as part of adding support for end to end testing to BuddyPress. We have a lot of prior art, including from WordPress core ​here and ​here.

My initial suggestion would be to add a new Github Action with e2e tests for the core steps of this new checklist (it should not be too extensive to begin with), but we can improve upon it as time goes by. This would create the base for getting started with end‑to‑end performance testing in BuddyPress in the future, where we can test if a new release is more or less performant.

[espellcaste]

I'll handle this in the next milestone considering our current goals.