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

Fix string_tests.py::MixinStrUnicodeTest.test_bug1001011():
Test str and str8.
diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 53a3ddb..d5a9a29 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -1154,7 +1154,7 @@
             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)