Add iOS iPad UI tests to engine scenario tests

Add a Scenario app test scheme for iPad to facilitate testing [iPadOS pointer interactions](https://developer.apple.com/design/human-interface-guidelines/ios/user-interaction/pointers/) like https://github.com/flutter/engine/pull/27019.