Port all string tests to PyUnit and share as much tests

between str, unicode, UserString and the string module
as possible. This increases code coverage in stringobject.c
from 83% to 86% and should help keep the string classes
in sync in the future. From SF patch #662807

Lib/test/test_str.py

@@ -0,0 +1,23 @@
+import unittest
+from test import test_support, string_tests
+
+
+class StrTest(
+    string_tests.CommonTest,
+    string_tests.MixinStrUnicodeUserStringTest,
+    string_tests.MixinStrUserStringTest
+    ):
+
+    type2test = str
+
+    # We don't need to propagate to str
+    def fixtype(self, obj):
+        return obj
+
+def test_main():
+    suite = unittest.TestSuite()
+    suite.addTest(unittest.makeSuite(StrTest))
+    test_support.run_suite(suite)
+
+if __name__ == "__main__":
+    test_main()