test: Add rpcauth pair that generated by rpcauth

Summary:

This PR adds a rpcauth pair that is randomly generated. Also checks that
rpcauth.py works fine.

Backport of core PR13024
https://github.com/bitcoin/bitcoin/pull/13024/files

Test Plan:

./test/functional/test_runner.py rpc_users

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3758