bench: use the node context & chain from the TestSetup instead of creating a new one

Summary:
I missed this change when backporting core#19425 in D11419. It caused an assertion to fail when running this benchmark:

bitcoin-bench: ../../src/interfaces/chain.cpp:226: interfaces::{anonymous}::ChainImpl::findBlock(const BlockHash&, const interfaces::FoundBlock&)::<lambda()>: Assertion `"m_node.chainman" &&check' failed.

Link to original Bitcoin Core commit

Test Plan: ninja bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11430