test: add multiwallet tests for bitcoin-cli -generate

Summary:
This is a backport of core#19133 [9/10]
https://github.com/bitcoin/bitcoin/pull/19133/commits/bf53ebef061a563cfc4c5857f5d6bc93fb136282

Depends on D9948

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9949