[2.7] bpo-30223: Add Lib/test/__main__.py. (#1373)

To unify running tests in Python 2.7 and Python 3, the test
package can be run as a script.  This is equivalent to running the
test.regrtest module as a script.
diff --git a/Lib/test/pickletester.py b/Lib/test/pickletester.py
index 138e17e..4268394 100644
--- a/Lib/test/pickletester.py
+++ b/Lib/test/pickletester.py
@@ -1,4 +1,6 @@
 # -*- coding: utf-8 -*-
+from __future__ import absolute_import
+
 import unittest
 import pickle
 import cPickle