>> Why can't the filename of the "foo"-like file in the test be
>> simply os_helper.TESTFN, as done in some other tests?
> 
> I suppose the current working directory will be fine. I was looking to keep the test on a NTFS filesystem, with known behavior, but there's no hard guarantee that the user's temp directory is on the system volume.

All tests should use the current working directory, or the test helper 
for getting other directories. *Do not look up other directories*, 
because it prevents test runners from ensuring that tests run in valid 
locations (and skips implicit tests for directories with spaces/Unicode 
characters/etc.)