test: Skip flat-tree tests if devicetree is not used

Many tests don't actually use the devicetree at all so there is no point
in running the tests both with livetree and flat tree. Check for this and
skip the flat tree test in that case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>