test: rpc_users: Add function for auth'd requests.

Summary:
Straightfoward refactoring of code.

This is a partial backport of Core PR16334
Commit 604e2a997ff26202dd0fa1932d60dc14cc53ac6d

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/D7852