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>

test/dm/test-main.c

@@ -127,7 +127,7 @@ static bool dm_test_run_on_flattree(struct unit_test *test)
 	return !strstr(fname, "video") || strstr(test->name, "video_base");
 }
 
-static int dm_test_main(const char *test_name)
+int dm_test_main(const char *test_name)
 {
 	struct unit_test *tests = ll_entry_start(struct unit_test, dm_test);
 	const int n_ents = ll_entry_count(struct unit_test, dm_test);