New testing guide nits

[huonw]

• the benchmark + optimisations heading is useful Revamped testing guide #19627 (comment)
• overly optimised benchmark that's meant to be defeating the optimiser Revamped testing guide #19627 (comment)
• removed paragraph Revamped testing guide #19627 (comment)

Fixing this just means responding to/justifying/whatever each of those, not necessarily making changes to the guide.

cc @steveklabnik