test/py: log when tests send CTRL-C

Write a note to the log file when a test sends CTRL-C to U-Boot. This
makes it easier to follow what's happening in the logs, especially since
U-Boot doesn't echo the character back to its output, so there's no other
signal of what's going on.

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