[3.13] gh-129248: Filter out the iOS log prefix from testbed runner output. (GH-129252) (#129283)

Filter out the iOS log prefix from testbed runner output.
(cherry picked from commit a58083811a51764c47fbb7cbd67e87e1124d53e8)

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>