Add unittest for set next op index am: cfe694f9ac am: bdcf688606

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1498857

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Icedb3f1ea8f1921c24c65daee2ed3a0fc142da0d