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>

arch/sandbox/cpu/spl.c

@@ -65,7 +65,7 @@ void spl_board_init(void)
 	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 */
 	}
 }