tests: Add tests cases for the overlay code

Add some test infrastructure to test that the overlay can be merged, but
also that poorly formatted fixups would fail as expected.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
[dwg: Don't execute bad overlay tests without overlay aware dtc]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>