test: resolve bug in test/functional/interface_bitcoin_cli.py

Summary:
The test "Test -getinfo with -rpcwallet=unloaded wallet returns no balances" was not actually asserting anything useful as cli_get_info_keys was not defined and was reused from an earlier test instead.

This is a backport of core#21822

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13185