test/py: test that sandbox exits when asked

Test the sandbox port's implementation of the reset command and SIGHUP
handling. These should both cause the U-Boot process to exit gracefully.

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