[chronik] Fix chronik_shutdown.py

Summary:
Since D18712 the txid is no longer cached. In this test this is causing an issue as the hash computation is now causing the test to time out.
With this change we go back to expected duration: from 45s per iteration to 0,11s per iterations on my machine.
This issue was caught a bit late because the CI doesn't run the chonik extended tests outside of the timing file update, so let's fix this as well so we notice these issues quickly.

Test Plan:

./test/functional/test_runner.py chronik_shutdown

Check you didn't die before it completed.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18777