[3.13] Ensure that iOS test re-runs don't try to spawn a process. (GH-122994) (#123368)

Ensure that iOS test re-runs don't try to spawn a process. (GH-122994)

Adds the --single-process option to the iOS test runner to ensure re-runs execute in the same process.
(cherry picked from commit e03073ff20107793a4ea28cdac0d6894774dd110)

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