[camera_avfoundation] Remove unused Objective-C testing files

[cdeil]

Cleanup for flutter/flutter#119109 removing old unused files.

I ran into Pod-related error with the tests and saw that #11237 will resolve that, so this is just a draft that should wait for #11237 to land.

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[gemini-code-assist]

Code Review

This pull request removes unused Objective-C testing files (ExceptionCatcher.h, ExceptionCatcher.m, and RunnerTests-Bridging-Header.h) from the camera_avfoundation example. As part of this cleanup, references to these files are removed from the Xcode project file (project.pbxproj). Additionally, import Foundation statements are added to three Swift mock files that required it after the removal of the bridging header.

[stuartmorgan-g]

LGTM