snow: Enable unit tests

It is useful to have a board with unit tests enabled, to check that this
does not break.

Let's choose snow, since it is not under active development and it is
glorious.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>