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>
diff --git a/test/image/Kconfig b/test/image/Kconfig
index 963c86c..a52766b 100644
--- a/test/image/Kconfig
+++ b/test/image/Kconfig
@@ -14,12 +14,13 @@
 	bool "Unit tests for filesystems"
 	depends on SANDBOX && SPL_OF_REAL
 	depends on FS_LOADER
+	depends on SPL_BLK_FS
 	depends on SPL_FS_FAT
 	depends on SPL_FS_EXT4
 	depends on SPL_MMC_WRITE
 	default y
 	help
-	  Test filesystems in SPL.
+	  Test filesystems and the various load methods which use them.
 
 config SPL_UT_LOAD_OS
 	bool "Test loading from the host OS"