Enable check_untyped_defs mypy option for testing/ too

testing/example_scripts/dataclasses/test_compare_dataclasses.py

@@ -2,11 +2,11 @@ from dataclasses import dataclass
 from dataclasses import field
 
 
-def test_dataclasses():
+def test_dataclasses() -> None:
     @dataclass
     class SimpleDataObject:
         field_a: int = field()
-        field_b: int = field()
+        field_b: str = field()
 
     left = SimpleDataObject(1, "b")
     right = SimpleDataObject(1, "c")