Remove init of global config mid-test in miner_tests.cpp

Summary: Removes the possibility of accidentally using two different configs, which can cause these tests fail. Discovered while working on D1519, although that diff will not be applied, the issue is still valid.

Test Plan: make check

Reviewers: deadalnix, Mengerian, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1732