Fix bench/bench_bitcoin (Select REGTEST Params before wallet init)
Summary: globalChainParams needs to be created by SelectParams before it can be used in bench/coin_selection.cpp
Test Plan: bench_bitcoin doesn't crash any longer
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Subscribers: teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1787