sandbox: Allow selection of SPL unit tests

Now that we have more than one test, add a way to select the test to run.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/spl.c b/arch/sandbox/cpu/spl.c
index 48fd126..81b217a 100644
--- a/arch/sandbox/cpu/spl.c
+++ b/arch/sandbox/cpu/spl.c
@@ -72,7 +72,7 @@
 	if (state->run_unittests) {
 		int ret;
 
-		ret = dm_test_main(NULL);
+		ret = dm_test_main(state->select_unittests);
 		/* continue execution into U-Boot */
 	}
 }