Robustify sub_test when os errors occur during unlinking.

Ignore errors when deleting the executable during testing.
There was already an exception block that was ignoring IOErrors in sub_test
when trying to unlink the executable. Update the except block to also catch
OSErrors.

Also add lsof/handle call when "Device or resource busy" error occurs. This
will help debug the core issue that is now swept under the covers (see
above paragraph).