Add test for new __ne__ method on Promise.

tests/utils_tests/test_functional.py

@@ -64,3 +64,15 @@ class FunctionalTestCase(unittest.TestCase):
 
         # check that it behaves like a property when there's no instance
         self.assertIsInstance(A.value, cached_property)
+
+    def test_lazy_equality(self):
+        """
+        Tests that == and != work correctly for Promises.
+        """
+
+        lazy_a = lazy(lambda: 4, int)
+        lazy_b = lazy(lambda: 4, int)
+        lazy_c = lazy(lambda: 5, int)
+
+        self.assertEqual(lazy_a(), lazy_b())
+        self.assertNotEqual(lazy_b(), lazy_c())