owtestlearners: Show correct error

Author: lanzagar

Orange/widgets/evaluate/owtestlearners.py

@@ -355,7 +355,7 @@ def set_test_data(self, data):
             self.Error.test_data_empty()
             data = None
         if data and not data.domain.class_var:
-            self.Error.class_required()
+            self.Error.class_required_test()
             data = None
         else:
             self.Error.class_required_test.clear()