add debugging code to the failing `drop_cache_rather_than_verify` test to get more analysis hints

src/ZEO/tests/drop_cache_rather_than_verify.txt

@@ -75,6 +75,10 @@ Now, we'll restart the server on the original address:
 
     >>> wait_connected(db.storage)
 
+##### debugging only ########
+    >>> print(db.storage._server.client.verify_result)
+    cache too old, clearing
+
 Now, let's verify our assertions above:
 
 - Publishes a stale-cache event.