Update tests/run-make/unstable-feature-usage-metrics/rmake.rs

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

tests/run-make/unstable-feature-usage-metrics/rmake.rs

@@ -51,9 +51,9 @@ fn test_metrics_dump() {
         let json_path =
             metrics.pop().expect("there should be one metrics file in the output directory");
 
-        assert_eq!(
-            0,
-            metrics.len(),
+        // After the `pop` above, there should be no files left.
+        assert!(
+            metrics.is_empty(),
             "there should be no more than one metrics file in the output directory"
         );