Adjust unit tests for supporting chain partition with no ab am: e41e464abb

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2765271

Change-Id: Ie66ac57dd849c5fc6b0f4fcedcc8b7a45ec8c39b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>