Add basic tests for the return value of os.popen().close().

According to #6358, python 3.0 has a different implementation that behaves differently.

Lib/test/test_popen.py

@@ -40,6 +40,13 @@ class PopenTest(unittest.TestCase):
         )
         test_support.reap_children()
 
+    def test_return_code(self):
+        self.assertEqual(os.popen("exit 0").close(), None)
+        if os.name == 'nt':
+            self.assertEqual(os.popen("exit 42").close(), 42)
+        else:
+            self.assertEqual(os.popen("exit 42").close(), 42 << 8)
+
 def test_main():
     test_support.run_unittest(PopenTest)