[chronik] Fix flakiness in chronik_script_history

Summary: Make sure chronik processed the block before checking its txs.

Test Plan:

./test/functional/test_runner.py chronik_script_history

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17494