Some simplification in the compact block code and tests

Summary: As per title.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, sickpig, kyuupichan, awemany, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D232