test: spl: Add a test for the MMC load method

Add a test for the MMC load method. This shows the general shape of tests
to come: The main test function calls do_spl_test_load with an appropriate
callback to write the image to the medium.

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