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/start.c

@@ -374,6 +374,15 @@ static int sandbox_cmdline_cb_show_of_platdata(struct sandbox_state *state,
 }
 SANDBOX_CMDLINE_OPT(show_of_platdata, 0, "Show of-platdata in SPL");
 
+static int sandbox_cmdline_cb_unittests(struct sandbox_state *state,
+					const char *arg)
+{
+	state->run_unittests = true;
+
+	return 0;
+}
+SANDBOX_CMDLINE_OPT_SHORT(unittests, 'u', 0, "Run unit tests");
+
 static void setup_ram_buf(struct sandbox_state *state)
 {
 	/* Zero the RAM buffer if we didn't read it, to keep valgrind happy */