test: add tests for bitcoin-cli -generate

Summary:
This is a backport of core#19133 [7/10]
https://github.com/bitcoin/bitcoin/pull/19133/commits/18f93545a12db00180cea369a4b5cce7f10cd362

Depends on D9946

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9947