test: Rename test-main.c to test-dm.c

This is the main test function for driver model but not for other tests.
Rename the file and the function so this is clear.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/spl.c b/arch/sandbox/cpu/spl.c
index 6926e24..3779d58 100644
--- a/arch/sandbox/cpu/spl.c
+++ b/arch/sandbox/cpu/spl.c
@@ -65,7 +65,7 @@
 	if (state->run_unittests) {
 		int ret;
 
-		ret = dm_test_main(state->select_unittests);
+		ret = dm_test_run(state->select_unittests);
 		/* continue execution into U-Boot */
 	}
 }