Rename and move test framework constant to MAX_BLOCK_SIGOPS_PER_MB

Summary:
Still more test preparation

Nothing fancy, just get MAX_BLOCK_SIGOPS out of the test, renamed to indicate
that it is per MB of blocksize, and move it into into mininode.py to live with
other framework-wide constants until we find them a better home.

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

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D155