[Vertex AI] Add test app for integration tests

[andrewheard]

• Added a test app to host Vertex AI in Firebase integration tests.
  • This approach provides keychain access, which allows us to use App Check with the Debug Provider.
• Added tests to check the Firebase Auth / Storage integration.

#no-changelog

[paulb777]

This is great!

My only concern is that I don't see any tests in the log at https://github.com/firebase/firebase-ios-sdk/actions/runs/11525405469/job/32088217991

[paulb777]

Maybe the minimum iOS version needs to be set on the xcodebuild command?

[andrewheard]

My only concern is that I don't see any tests in the log at firebase/firebase-ios-sdk/actions/runs/11525405469/job/32088217991

Interestingly it only seems to produce output when I intentionally make the tests fail (e.g., this previous run). I was also able to see the calls to App Check in the Firebase Console for this CI run (they now show verified).

Maybe the minimum iOS version needs to be set on the xcodebuild command?

I'll dig into why the tests aren't listed one by one when they succeed in a follow-up PR. I'll also drop the deployment target to iOS 15 (it's unnecessarily high at 17.2 -- I forgot to change the default in the new Xcode project).