Ensure config is resetted with each tests.

Summary:
This fixes problems where there are leftover cofngis from other tests.

The long term plan would be to make sure the config is not a global singleton but we are not there yet.

Test Plan:

./src/test/test_bitcoin --random=13174

Reviewers: freetrader, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D351