test: Add framework to test hush behavior

test: Add framework to test hush behavior

Introduce a new subcommand to ut: ut hush.
For the moment, this command does nothing, future commits will add tests which
will be run on command call.

Note that CONFIG_HUSH_PARSER must be defined to compile this new subcommand.

Signed-off-by: Francis Laniel <francis.laniel@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>