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>

include/test/test.h

@@ -94,4 +94,15 @@ enum {
 	TEST_DEVRES_SIZE3	= 37,
 };
 
+/**
+ * dm_test_main() - Run driver model tests
+ *
+ * Run all the available driver model tests, or a selection
+ *
+ * @test_name: Name of single test to run (e.g. "dm_test_fdt_pre_reloc" or just
+ *	"fdt_pre_reloc"), or NULL to run all
+ * @return 0 if all tests passed, 1 if not
+ */
+int dm_test_main(const char *test_name);
+
 #endif /* __TEST_TEST_H */