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/test_str.py b/Lib/test/test_str.py
index be26b24..6fa3779 100644
--- a/Lib/test/test_str.py
+++ b/Lib/test/test_str.py
@@ -9,7 +9,7 @@
     string_tests.MixinStrUnicodeTest,
     ):
 
-    type2test = str
+    type2test = str8
 
     # We don't need to propagate to str
     def fixtype(self, obj):