Fail if a test leaves the browser logged in

What does this MR do?

All QA tests expect to be able to log in at the start of the test.

That's not possible if a test leaves the browser logged in when it finishes. Normally this isn't a problem because Capybara resets the session after each test. But it does that in an after block, so when a test logs in in an after(:all) block, the browser returns to a logged in state after Capybara has logged it out. And then the next test will fail.

E.g.: #34736 (closed)

This adds a config.after(:context) block to fail if a test leaves the browser logged in, and updates the best practices doc.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content