dm: test: Add a way to run SPL tests

Add a -u flag for U-Boot SPL which requests that unit tests be run. To
make this work, export dm_test_main() and update it to skip test features
that are not used with of-platdata.

To run the tests:

   $ spl/u-boot-spl -u
   U-Boot SPL 2020.10-rc5 (Oct 01 2020 - 07:35:39 -0600)
   Running 0 driver model tests
   Failures: 0

At present there are no SPL unit tests.

Note that there is one wrinkle with these tests. SPL has limited memory
available for allocation. Also malloc_simple does not free memory
(free() is a nop) and running tests repeatedly causes driver-model to
reinit multiple times and allocate memory. Therefore it is not possible
to run more than a few tests at a time. One solution is to increase the
amount of malloc space in sandbox_spl. This is not a problem for pytest,
since it runs each test individually, so for now this is left as is.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/spl.c

@@ -12,6 +12,7 @@
 #include <spl.h>
 #include <asm/spl.h>
 #include <asm/state.h>
+#include <test/test.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 
@@ -67,6 +68,13 @@ void spl_board_init(void)
 		     uclass_next_device(&dev))
 			;
 	}
+
+	if (state->run_unittests) {
+		int ret;
+
+		ret = dm_test_main(NULL);
+		/* continue execution into U-Boot */
+	}
 }
 
 void __noreturn jump_to_image_no_args(struct spl_image_info *spl_image)