Use ONE_MEGABYTE where historic limit is needed, extend some tests

Summary:
Some preparatory work to make the code easier to understand:

• Use ONE_MEGABYTE in place of DEFAULT_MAX_BLOCK_SIZE where the old 1MB limit is definitely needed
• extend some tests where ONE_MEGABYTE should be tested distinct from DEFAULT_MAX_BLOCK_SIZE and some other missing cases

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D137