bpo-35412: Add testcase to test_future4 (GH-11131)

Add testcase to test_future4: check unicode literal.
diff --git a/Lib/test/test_future4.py b/Lib/test/test_future4.py
index 413dd4d..b27ca40 100644
--- a/Lib/test/test_future4.py
+++ b/Lib/test/test_future4.py
@@ -1,6 +1,11 @@
 from __future__ import unicode_literals
-
 import unittest
 
+
+class Tests(unittest.TestCase):
+    def test_unicode_literals(self):
+        self.assertIsInstance("literal", str)
+
+
 if __name__ == "__main__":
     unittest.main()