Some other tests, when failing, don't always remove their TESTFN file.

Try to do it for them, so our mkdir() operation doesn't fail.

Lib/test/test_mailbox.py

@@ -2,6 +2,12 @@ import mailbox
 import os
 import test_support
 
+# cleanup
+try:
+    os.unlink(test_support.TESTFN)
+except os.error:
+    pass
+
 # create a new maildir mailbox to work with:
 curdir = os.path.join(test_support.TESTFN, "cur")
 newdir = os.path.join(test_support.TESTFN, "new")