test/py: add DFU test

Add a test of DFU functionality to the Python test suite. The test
starts DFU in U-Boot, waits for USB device enumeration on the host,
executes dfu-util multiple times to test various transfer sizes, many
of which trigger USB driver edge cases, and finally aborts the DFU
command in U-Boot.

This test mirrors the functionality previously available via the shell
scripts in test/dfu, and hence those are removed too.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>