Test LoadExternalBlockFile explictly for blocks larger than 2*MAX_TX_SIZE

Summary:
This checks that the buffering logic in CBufferedFile/LoadExternalBlockFile
works as intended and doesn't fail for blacks larger than the buffer size.

Test Plan: make check

Reviewers: deadalnix, freetrader, sickpig, kyuupichan, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D189