test: add -getinfo multiwallet functional tests

Summary:
and improve the existing -getinfo -rpcwallet tests.

This concludes the backport of Core PR18594 [6/6]
https://github.com/bitcoin/bitcoin/pull/18594/commits/5edad5ce5d3f15b694bf3fad0300c6446674b554

Depends on D9241

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9242