test: rpc_users: Also test rpcauth.py with specified password.

Summary:
This is a partial backport of Core PR16334
Commit 830dc2dd0fccb7f3ec49ff7233a188d92c541e7e

Depends on D7853

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/D7856