test: show filename of missing file, if test fails

In ethportal-peertest, it showed that a file was missing, but not which
one. That information is critical, and self_peertest runs slowly, so
reducing reruns is very valuable.

ethportal-peertest/src/utils.rs

@@ -105,7 +105,7 @@ fn read_history_content_key_value(
 /// Wrapper function for fixtures that directly returns the tuple.
 fn read_fixture(file_name: &str) -> (HistoryContentKey, HistoryContentValue) {
     read_history_content_key_value(file_name)
-        .unwrap_or_else(|err| panic!("Error reading fixture: {err}"))
+        .unwrap_or_else(|err| panic!("Error reading fixture in {file_name}: {err}"))
 }
 
 /// History HeaderWithProof content key & value