[minig] make CBlockAssembler take a mempool for unit testing.

Summary:
Start passing in mempool into CBlockAssembler for easier unit testing.

Depends on D2058

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2048