Rename all 1MB limits in qa tests to LEGACY_MAX_BLOCK_BASE_SIZE

Summary:
This is prep work so that we know where we need to review and adapt in qa tests,
as we roll out >1MB capability.

Test Plan: qa/pull-tester/rpc-tests -extended

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D153