dm: core: Run tests with both livetree and flat tree

Some tests require either livetree or flat tree. Add flags to allow the
tests to specify this. Adjust the test runner to run with livetree (if
supported) and then flat tree.

Some video tests are quite slow and running on flat tree adds little extra
test value, so run these on livetree only.

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