test: dm: test.dts - move to sandbox dts directory

The file test.dts from driver model test directory,
was compiled by call dtc in script: test/dm/test-dm.sh.

This doesn't allow for including of dtsi files and using
of C preprocessor routines in this dts file.

Since the mentioned script builds U-Boot before tests,
then moving the test.dts file into sandbox dts directory
is reasonable.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested on sandbox:
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/test-main.c b/test/dm/test-main.c
index a47bb37..7348f69 100644
--- a/test/dm/test-main.c
+++ b/test/dm/test-main.c
@@ -78,8 +78,7 @@
 	 */
 	if (!gd->fdt_blob || fdt_next_node(gd->fdt_blob, 0, NULL) < 0) {
 		puts("Please run with test device tree:\n"
-		     "     dtc -I dts -O dtb test/dm/test.dts  -o test/dm/test.dtb\n"
-		     "    ./u-boot -d test/dm/test.dtb\n");
+		     "    ./u-boot -d arch/sandbox/dts/test.dtb\n");
 		ut_assert(gd->fdt_blob);
 	}