cli -getinfo: use getbalances instead of deprecated getwalletinfo balance

Summary:

This PR updates bitcoin-cli -getinfo to fetch the wallet balance from getbalances in order to no longer depend on getwalletinfo.balance which was deprecated a year ago in D6447

test: update and harden interface_bitcoin_cli tests

• no longer depend on the deprecated balance field of getwalletinfo
• test blockcount and balance against non-default, non-zero expected values

This is a backport of Core PR18574

Test Plan:
ninja all check-all
src/bitcoin-cli -getinfo

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8912