[tests] Dynamically detect tvOS simulator device name
The tvOS branch of `get_simulator_command` hardcoded `'Apple TV'`, but
Xcode 27 no longer registers a default device with that exact name --
only `Apple TV 4K (3rd generation)` (and `(at 1080p)`) are created --
so `simctl spawn --standalone 'Apple TV'` fails with `Invalid device`
on hosts where nothing has recreated the older name. Detect the device
dynamically via `simctl list`, mirroring the treatment iOS and watchOS
got in #89894.
