[3.13] gh-153631: Ensure iOS simulator is running before starting build (GH-153632)

Makes a small change to the iOS build script to improve build stability.
xcodebuild doesn't guarantee that the CoreSimulatorService is running before
starting a simulator. If the service isn't running, xcodebuild reports that no
simulators are available, and fails to start the test app. However, simctl
blocks until the simulator is available, and simctl is used to evaluate the
default simulator. So - the iOS build script now unconditionally determines the
default simulator, even if a specific simulator is requested.
(cherry picked from commit 2dc1a91)

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

Author: freakboy3742

iOS/testbed/__main__.py

@@ -220,8 +220,14 @@ def run_testbed(simulator: str | None, args: list[str], verbose: bool = False):
     update_test_plan(location, args)
     print(" done.")
 
+    # xcodebuild doesn't guarantee that the CoreSimulatorService daemon is
+    # running prior to using a simulator, but calling `xcrun simctl list` does.
+    # Determining the default simulator that *would* be used is a cheap action;
+    # so use that as a way to ensure that the CoreSimulatorService daemon is
+    # running.
+    default_simulator = select_simulator_device()
     if simulator is None:
-        simulator = select_simulator_device()
+        simulator = default_simulator
     print(f"Running test on {simulator}")
 
     xcode_test(location, simulator=simulator, verbose=verbose)