test: py: Extend fpga test with fit image with external data

Images are created
mkimage -f fit.its -E  download-fit-external.ub

and test expects these entries.

env__fpga_under_test = {
    ...
    "mkimage_fit_external": download-fit-external.ub",
    "mkimage_fit_external_size": xxxxx,
    ...
}

Test download file and loads it to fpga.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>