test/py: dfu: error out if USB device already exists

The DFU test requests U-Boot configure its USB controller in device mode,
then waits for the host machine to enumerate the USB device and create a
device node for it. However, this wait can be fooled if the USB device
node already exists before the test starts, e.g. if some previous software
stack already configured the USB controller into device mode and never
de-configured it. This "previous software stack" could even be another
test/py test, if U-Boot's own USB teardown does not operate correctly. If
this happens, dfu-util may be run before U-Boot is ready to serve DFU
commands, which may cause false test failures.

Enhance the dfu test to fail if the device node exists before it is
expected to.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>