fix test_site/test_startup_imports when some of the extensions are built as builtins.

--- a/Lib/test/test_site.py	Mon Apr 14 12:24:37 2014 -0400
+++ b/Lib/test/test_site.py	Mon Apr 14 22:17:57 2014 +0200
@@ -459,7 +459,8 @@
         # http://bugs.python.org/issue19218>
         collection_mods = {'_collections', 'collections', 'functools',
                            'heapq', 'itertools', 'keyword', 'operator',
-                           'reprlib', 'types', 'weakref'}
+                           'reprlib', 'types', 'weakref'
+                          }.difference(sys.builtin_module_names)
         self.assertFalse(modules.intersection(collection_mods), stderr)

the test now passes indepedent of the status of _collections (builtin or extension).