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