[backport#14845] [tests] Add wallet_balance.py

Summary:
Adds a test specifically to test the wallet's getbalance and
getunconfirmedbalance RPCs.

This is a backport of Core PR14845

Test Plan:

cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug
./test/functional/test_runner,py wallet_balance wallet_basic

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5950