Use Xcode 11.3.0 and iOS 13.3 in tests (#27783)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/27783

Use Xcode 11.3.0 to run tests, using an iPhone 8 simulator running iOS 13.3.

Snapshot tests disabled as they're failing on Circle CI.

Changelog:
[iOS] [Changed] - Use Xcode 11.3 and iOS 13.3 simulator in iOS tests

Reviewed By: PeteTheHeat

Differential Revision: D19411972

fbshipit-source-id: 3ddef9e6fbdbd3c35271732fc6d6d74de099230e