[qa] move and rename mine_large_block from util to blocktools

Summary: As per title. Depends on D1634

Test Plan: ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1636