handle no warning

Author: TomAugspurger

pandas/util/testing.py

@@ -2651,8 +2651,11 @@ class for all warnings. To check that no warning is returned,
                     pass
 
         saw_warning = False
-        if filter_level:
+        if expected_warning and filter_level:
             warnings.filterwarnings(filter_level, message, expected_warning)
+        elif filter_level:
+            # no expected warnings.
+            warnings.simplefilter(filter_level)
         yield w
         extra_warnings = []