test: nits in TestChain100Setup

Summary:

test: Move doxygen comment to header

test: Pass empty tx pool to block assembler

This is a partial backport of core#19775, without the last commit (segwit related)

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10127