tests: test_dfu.py: Add example udev rule for host_usb_dev_node

tests: test_dfu.py: Add example udev rule for host_usb_dev_node

If one does not already have a rule to create a custom device node when
a given device enumerates it can be useful to have udev create a
bus path based node to the entry in /dev/bus/usb that was just
enumerated.  Given that DFU itself does not require a /dev entry it is a
good idea to provide a rule that will generate one.

Signed-off-by: Tom Rini <trini@konsulko.com>