Fix Xcode 16 iOS build issues (#22379)
- Work around Xcode 16 iOS test build issue: `error: Multiple commands produce '.../PlugIns'`.
- Fix link error in iOS static framework test.
- Update build.py to check for the right kind of build before running iOS tests on the simulator.
- Update Xcode 16 build images to 'macos-15' because that's the only image that will have Xcode 16 soon. See https://github.com/actions/runner-images/issues/10703.
