of: unittest: overlay: Keep track of created overlays

During the course of the overlay selftests some of them remain
applied. While this does not pose a real problem, make sure you track
them and destroy them at the end of the test.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Rob Herring <robh@kernel.org>