Fix platform argument

[tokou]

Proposed changes

• Fix issue with platform not working for iOS
  • This was due to selectDevice being passed the platform as well as a deviceId corresponding to the first available device
  • The fix is to filter available devices on the given platform if passed. Which makes sense
• Add test specific platform and device argument in order to facilitate commands like maestro test flow.yml --platform android which did not work before

Testing

• Run commands with 1 android and 1 iOS device running
  • maestro test flow.yml --platform android
  • maestro test flow.yml --platform ios
  • maestro --platform android test flow.yml
  • maestro --platform ios test flow.yml
  • maestro --platform android test flow.yml --platform ios
  • maestro test flow.yml --platform ios --device emulator-5554
  • maestro test flow.yml

Issues fixed

Fixes #2037