[MINOR] Follow-up from "RPC Concurrency Test"

MINOR MERGE REQUEST

Overview

This MR

• makes all required fields static on the renterHostPair
• makes sure its methods are not unnecessarily exported
• makes sure its methods follow our naming conventions

I marked it minor because it's limited to naming (and alphabetizing) methods of a helper struct. It only touches test files.

Issues Closed

Closes #4105 (closed)