Refactor the imports in mining_* tests

Summary:
Remove all the import *, fix imports from messages and make the layout
consistent across tests.
This will later allow for more checks using flake8.
There is no functional change.

Test Plan:

./test/functional/test_runner.py mining_*

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2595