test: Move the devicetree check into ut_run_list()

test: Move the devicetree check into ut_run_list()

Add a check to ut_run_list() as to whether a list has driver model tests.
Move the logic for the test devicetree into that function, in an effort
to eventually remove all logic from dm_test_run().

Signed-off-by: Simon Glass <sjg@chromium.org>