explorerd: guard test_utils module with #[cfg(test)]

Adds a `#[cfg(test)]` attribute to the `test_utils` module in `main.rs` to ensure it is only included during testing.

bin/explorer/explorerd/src/main.rs

@@ -54,6 +54,7 @@ mod store;
 mod error;
 
 /// Test utilities used for unit and integration testing
+#[cfg(test)]
 mod test_utils;
 
 const CONFIG_FILE: &str = "explorerd_config.toml";