[3.14] gh-135101: When choosing the default simulator device, don't use `simctl --set testing` (GH-135102) (#135113)

On a fresh Xcode install (including some CI provider configurations), there is
no pre-existing testing set that can be used to identify simulator models. Use
the default device set to detect available models instead. Live testing
simulators are still created in the testing set.
(cherry picked from commit dba9de731b231ca0c079205f496d1e3d178b4fd3)

Co-authored-by: Joe Rickerby <joerick@mac.com>