Extract important parameters from sources outside the test framework

Summary:
We want things like DEFAULT_MAX_BLOCK_SIZE and the sigops limits to be
available to the test framework without duplicating them.

While we are doing this, move MAX_BLOCK_SIGOPS_PER_MB out of mininode.

Test Plan:
make check
qa/pull-tester/rpc-tests.py -extensive

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D157