test: Drop struct dm_test_state

Driver model is a core part of U-Boot. We don't really need to have a
separate test structure for the driver model tests and it makes it harder
to write a test if you have to think about which type of test it is.

Subsume the fields from struct dm_test_state into struct unit_test_state
and delete the former.

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