Add TestCase.longMessage to stdlib/2/unittest.pyi (#1875)

Fixes #1874.

stdlib/2/unittest.pyi

@@ -53,6 +53,7 @@ class _AssertRaisesContext(_AssertRaisesBaseContext):
 
 class TestCase(Testable):
     failureException = ...  # type: Type[BaseException]
+    longMessage = ...  # type: bool
     def __init__(self, methodName: str = ...) -> None: ...
     def setUp(self) -> None: ...
     def tearDown(self) -> None: ...