bench: Use existing NodeContext and chainman
Summary:
This is a backport of core#21767 [2 & 3 / 5]
https://github.com/bitcoin/bitcoin/pull/21767/commits/91226eb91769aad5a63bc671595e1353a2b2247a
https://github.com/bitcoin/bitcoin/pull/21767/commits/f4a47a1febfa35ab077f2a841fe31a8cd9618250
Test Plan: ninja bench-bitcoin
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D11633