Fix test_str.py so that it tests the str8 class.

Fix string_tests.py::MixinStrUnicodeTest.test_bug1001011():
Test str and str8.

Lib/test/string_tests.py

@@ -1154,7 +1154,7 @@ class subclass(t):
             s2 = "".join([s1])
             self.assert_(s1 is s2)
 
-        elif t is str:
+        elif t is str8:
             s1 = subclass("abcd")
             s2 = "".join([s1])
             self.assert_(s1 is not s2)

Lib/test/test_str.py

@@ -9,7 +9,7 @@ class StrTest(
     string_tests.MixinStrUnicodeTest,
     ):
 
-    type2test = str
+    type2test = str8
 
     # We don't need to propagate to str
     def fixtype(self, obj):