[3.13] gh-129200: Add locking to the iOS testbed startup sequence. (GH-130564) (#130657)

Add a lock to ensure that only one iOS testbed per user can start at a time, so
that the simulator discovery process doesn't collide between instances.
(cherry picked from commit 9211b3dabeacb92713ac3f5f0fa43bc7cf69afd8)

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