Fix ThreadStress check script to filter stderr. am: 649beaaead am: 9f9c55f4f2

Original change: https://android-review.googlesource.com/c/platform/art/+/1572461

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ia440e761dacf63d6ca170a8e900a29062a9bccf1