ci: wait for the test simulator before running xcodebuild - #557

The Test and Test (ASan) jobs sometimes burn their first "Xcode Tests" attempt in about a minute, without building anything:

xcodebuild: error: Unable to find a device matching the provided destination specifier:
		{ platform:iOS Simulator, OS:latest, name:iPhone 16 Pro }
...
Warning: Attempt 1 failed. Reason: Child_process exited with error code 70

On a fresh runner, CoreSimulator can still be enumerating its devices when the step starts, so the destination doesn't resolve yet. The retry, about 50 s later, finds the device and passes. This happened on both test jobs of #547 and on the ASan job of feat/ios-stdio-forwarding, so it isn't tied to any one change.

Change

• scripts/wait-for-simulator.sh --name <device> [--timeout <s>] polls xcrun simctl list devices available until an iOS device with that name is listed. A failing simctl call while CoreSimulator starts counts as "not yet". After the timeout (300 s by default) it fails with an ::error:: annotation and the device list.
• The test job runs it as "Wait for the test simulator" right before "Xcode Tests", with the same device name as the -destination.

It doesn't pre-boot the simulator. That would compete with the build on the runner's few cores, and the race is only about the device being listed.

Testing

• Ran the script locally:
  • with an existing device name it returns at once;
  • with a missing name it times out with exit 1, the annotation and the device list;
  • it rejects a missing --name, a non-numeric --timeout and unknown flags.
• Both --flag value and --flag=value forms work.
• On this PR's first CI run the step found the simulator after 28 s (Test) and 125 s (Test (ASan)), and "Xcode Tests" passed on its first attempt in both jobs. The 125 s is why the default is 300 s rather than 180 s.