[3.13] gh-127845: Minor improvements to iOS test runner script (GH-127846) (#127892)

Uses symlinks to install iOS framework into testbed clone, adds a verbose mode
to the iOS runner to hide most Xcode output, adds another mechanism to disable
terminal colors, and ensures that stdout is flushed after every write.
(cherry picked from commit ba2d2fda93a03a91ac6cdff319fd23ef51848d51)

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