test: spl: Add a test for spl_blk_load_image

Add a test for spl_blk_load_image, currently used only by NVMe. Because
there is no sandbox NVMe driver, just use MMC instead. Avoid falling back
to raw images to make failures more obvious.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>