tests: overlay: Rename the device tree blobs to be more explicit

Rename the blobs to have a more explicit output that will give us a clearer
idea about whether a DT (and the test) has been compiled using a dtc with
our without overlays support.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>