Move test_bug1001011() to string_tests.MixinStrUnicodeTest so that
it can be used for str and unicode. Drop the test for
   "".join([s]) is s
because this is an implementation detail (and doesn't work for unicode)
diff --git a/Lib/test/test_str.py b/Lib/test/test_str.py
index 1295a8c..82632f1 100644
--- a/Lib/test/test_str.py
+++ b/Lib/test/test_str.py
@@ -5,7 +5,8 @@
 class StrTest(
     string_tests.CommonTest,
     string_tests.MixinStrUnicodeUserStringTest,
-    string_tests.MixinStrUserStringTest
+    string_tests.MixinStrUserStringTest,
+    string_tests.MixinStrUnicodeTest,
     ):
 
     type2test = str