[chronik] Fix clippy and restore the blockchain tests

Summary: D18237 was inadvertenly landed using the wrong revision, this diff restores the last changes.

Test Plan:

ninja check-crate-chronik-clippy
./test/functional/test_runner.py chronik_electrum_blockchain

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D18240