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

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>