Merge str.center tests from 3.3.

Lib/test/test_unicode.py

@@ -663,6 +663,15 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertEqual('ß'.swapcase(), 'SS')
         self.assertEqual('\u1fd2'.swapcase(), '\u0399\u0308\u0300')
 
+    def test_center(self):
+        string_tests.CommonTest.test_center(self)
+        self.assertEqual('x'.center(2, '\U0010FFFF'),
+                         'x\U0010FFFF')
+        self.assertEqual('x'.center(3, '\U0010FFFF'),
+                         '\U0010FFFFx\U0010FFFF')
+        self.assertEqual('x'.center(4, '\U0010FFFF'),
+                         '\U0010FFFFx\U0010FFFF\U0010FFFF')
+
     def test_contains(self):
         # Testing Unicode contains method
         self.assertIn('a', 'abdb')