The `file_data_filter` method should be called when files are deleted.
In this case the `data` and `file_ctx` keys map to None. This is so
that a filter which modifies file names can apply the same name
transformations before files are deleted.
It's included as a module for a reason.
Also, use "$0" so the tests can be run like `./t/main.t` (or any other
directory).
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
