test: rpc_users: Make variable names more clear.

Summary:
Remove username and password string duplication.

This concludes backport of Core PR16334.
Commit: e263a343d4b6a2622df6bb734cd9d51a0d20a663

Depends on D7856

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/D7857