test: rpc_users: Add function for testing auth params.

Summary:
Straightfoward refactoring of code.

This is a partial backport of Core PR16334
Commit c73d871799982ca29c29cef90e1a78814cf34019

Depends on D7852

Test Plan: ninja && ./test/functional/test_runner.py rpc_users

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7853