[tests] Make rpcauth.py testable and add unit tests

Summary:
Backport of core PR13056 and PR13073
https://github.com/bitcoin/bitcoin/pull/13056/files
https://github.com/bitcoin/bitcoin/pull/13073/files

Depends on D3758.

Test Plan:

make check
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3759