Backport leftovers from 15788

Summary:
test: Have segwit always active in (Basic)TestingSetup
test: Use test_bitcoin setup in bench, Add test utils

This is a partial bakport of Core PR15788 . the original backport missed many bits.

I did not port the RPC like method because they don't build for us and they are dead code.

Depends on D5848

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5849