Try to make this test more resistant to dropping from previous runs (ie, files...

Try to make this test more resistant to dropping from previous runs (ie, files that may exist but cause the test to fail).  Should be backported (assuming it works :-)

Lib/test/test_mailbox.py

@@ -54,6 +54,7 @@ class TestMailbox(TestBase):
 
     def setUp(self):
         self._path = test_support.TESTFN
+        self._delete_recursively(self._path)
         self._box = self._factory(self._path)
 
     def tearDown(self):