[test] Add test for getblockheader verboseness

Summary:
Add test for getblockheader with verbose argument set to false.

This is a backport of core#19722

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10120