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

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.

iOS/testbed/__main__.py

@@ -127,7 +127,7 @@ async def async_check_output(*args, **kwargs):
 async def select_simulator_device():
     # List the testing simulators, in JSON format
     raw_json = await async_check_output(
-        "xcrun", "simctl", "--set", "testing", "list", "-j"
+        "xcrun", "simctl", "list", "-j"
     )
     json_data = json.loads(raw_json)