[Part2] Use config.GetChainParams() insted of Params() where config is abailable in local scope

Summary: Replace direct calls of Params() with Config.GetChainParams() Refs T170

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

Reviewers: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Subscribers: teamcity

Maniphest Tasks: T170

Differential Revision: https://reviews.bitcoinabc.org/D1071