Fix ThreadStress check script to filter stderr. am: 649beaaead am: 9f9c55f4f2 am: 8d88287e6e

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I27d4b63698af24214278397fad5b7507fcff51ee