[chronik] test for the "Wiping existing scripthash index" log message

Summary:
Since we just check if the -chronikscripthashindex init flag is unset to return a 400 error, this test was not actually checking that index is wiped the first time the flag is unset. We could be returning that error even with the index still intact on disk.
We can at least check the presence of the relevant log line to verify that we hit the function that is supposed to wipe the index..

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17226