Improve TLS BR section 2.2 test website requirements

[wthayer]

A question on the list (thread) about chaining of test website certificates to roots was discussed during the 25-Sept 2025 SCWG call. There was general agreement that we should improve the language to:

• Clarify the purpose of test websites (e.g. are they needed after a root is included?)
• Clarify the answer to the original question
• Align with new root program requirements

[github-actions]

This issue was created based on:

• TLS BR Version 2.1.7
• EVG Version 2.0.1