test/py/tests/test_vboot.py: Add check that we boot the image

Make sure that when we're telling bootm to boot an image, and we expect
the image to boot we get the output from sandbox that we attempted to
run Linux and that U-Boot completed its job.

Cc: Simon Glass <sjg@chromium.org>
Cc: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>