Silence DeprecationWarnings in test_unicode.

Lib/test/test_unicode.py

@@ -2172,6 +2172,8 @@ class UnicodeTest(string_tests.CommonTest,
             # generate a fresh string (refcount=1)
             text = 'a' * length + 'b'
 
+            with support.check_warnings(('unicode_internal codec has been '
+                                         'deprecated', DeprecationWarning)):
                 # fill wstr internal field
                 abc = text.encode('unicode_internal')
                 self.assertEqual(abc.decode('unicode_internal'), text)