Add iOS 15 scenario images

[jmagman]

Add scenario app goldens for iOS 15. Always run on latest iOS so these tests will pass against versions of Xcode with different iOS SDKs available (as long as the images are checked in for the SDK).

Follow up to #30263
Part of flutter/flutter#85555.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[jmagman]

Closing, just going to wait for flutter/flutter#91689 and do this properly.