Convert run_ios_tests.sh to run_ios_tests.dart. (#53645)

Part of flutter/flutter#143953.

Because there are so many boundary cases, i.e. tests failed uploading artifacts to LUCI, I  was _not_ planning on atomically deleting and replacing `run_ios_tests.sh`, and want to give a chance for the iOS team to try the converted code and verify it works the way they expect.

I'm also happy to just slam it on, but defer to y'all. _It works on my machine_.

---

```sh
dart run testing/scenario_app/bin/run_ios_tests.dart
```

Author: matanlurey