Fix test_best_metric_for_not_well_defined_metric_collection

* Assign MetricTracker.best_metric return value to a tuple of the form (best, idx), instead of (idx, best).

tests/unittests/wrappers/test_tracker.py

@@ -170,7 +170,7 @@ def test_best_metric_for_not_well_defined_metric_collection(base_metric):
             assert best is None
 
     with pytest.warns(UserWarning, match="Encountered the following error when trying to get the best metric.*"):
-        idx, best = tracker.best_metric(return_step=True)
+        best, idx = tracker.best_metric(return_step=True)
 
         if isinstance(best, dict):
             assert best["MulticlassAccuracy"] is not None