scripts: add selftest layout to disk_layout_anthos.json

The 'selftest' layout is required when building kselftest images.
Add it to disk_layout_anthos.json so that boards inheriting from it
(anthos, arion, pholus) can successfully build kselftest images.

BUG=None
TEST=disk_layout_tool validate selftest src/private-overlays/overlay-arion-private/scripts/disk_layout.json
RELEASE_NOTE=None

Change-Id: I1a216ea3ffabb13f79dfabd3d11fcd7199131dc3
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/174167
Tested-by: Chenglong Tang <chenglongtang@google.com>
Reviewed-by: Angel Adetula <angeladetula@google.com>