[Chronik] Add tests for 404 errors on plugin endpoints

Summary: This is currently untested, and caused some confusion in D16605.

Test Plan: ./test/functional/test_runner.py chronik_plugins chronik_plugin_groups

Reviewers: Fabien, #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16609