Add test coverage for getblocktemplate's sigoplimit

Summary:
This value isn't expected to change for a while, but we should
test it anyway for sanity since we've already applied patches that
change its behavior in the past.

Part of T1014

Depends on D7233

Test Plan: test_runner.py abc_mining_basic

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7234