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test_mailbox fails if another failed test leaves a file or directory with the name given by support.TESTFN. The attached patch rectifies this, and also uses existing stdlib functionality (shutil.rmtree) to remove a directory tree recursively on tearDown.

Why not just call the helper in setUp? Otherwise, it looks good to me.
This is a bug fix and should be backported, I believe.

> test_mailbox fails if another failed test leaves a file or directory
> with the name given by support.TESTFN. The attached patch rectifies this
You should find which test left the file and fix that test instead.

> You should find which test left the file and fix that test instead.
Ideally yes, but it's easier said than done. When the failure occurred, there were many failures, and I couldn't locate the place where the file was being left behind. It was a while ago - I was remiss in not logging the issue then, but real life intervened :-(

Updated patch available on Rietveld - I'll commit it four hours from now if there are no objections.

New changeset 707586c70195 by Vinay Sajip in branch '3.2':
Closes #14158: improved resilience to test files left behind.
http://hg.python.org/cpython/rev/707586c70195
New changeset a92e73dfbff6 by Vinay Sajip in branch 'default':
Closes #14158: merged test file resilience fix from 3.2.
http://hg.python.org/cpython/rev/a92e73dfbff6 

Is 2.7 not affected?

> Is 2.7 not affected?
It might be, but the code for regrtest looked somewhat different. I will revisit this once the buildbots have run for a few days with the 3.x changes, to confirm that they are OK. Marking as pending to remind me.

New changeset 1112c2f602b3 by Vinay Sajip in branch '2.7':
Closes #14158: We now track test_support.TESTFN cleanup, and test_mailbox uses shutil.rmtree for simpler code.
http://hg.python.org/cpython/rev/1112c2f602b3