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>
diff --git a/configs/sandbox_noinst_defconfig b/configs/sandbox_noinst_defconfig
index 11be2dc..4f16d98 100644
--- a/configs/sandbox_noinst_defconfig
+++ b/configs/sandbox_noinst_defconfig
@@ -41,6 +41,8 @@
 CONFIG_SPL_HAS_CUSTOM_MALLOC_START=y
 CONFIG_SPL_CUSTOM_SYS_MALLOC_ADDR=0xa000000
 CONFIG_SPL_SYS_MALLOC_SIZE=0x4000000
+CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y
+CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x0
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SPL_I2C=y