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/include/asm/state.h

@@ -92,6 +92,7 @@ struct sandbox_state {
 	int default_log_level;		/* Default log level for sandbox */
 	bool show_of_platdata;		/* Show of-platdata in SPL */
 	bool ram_buf_read;		/* true if we read the RAM buffer */
+	bool run_unittests;		/* Run unit tests */
 
 	/* Pointer to information for each SPI bus/cs */
 	struct sandbox_spi_info spi[CONFIG_SANDBOX_SPI_MAX_BUS]