Restore iOS AvatarTests

[mischreiber]

Platforms Impacted

• iOS
• macOS

Description of changes

• Restoring AvatarTests.swift to our test package, updating the tests as needed for our new APIs.
• Updating Avatar class to use some internal structures instead of private, allowing for deeper testability.
• Enabling tests to run in the Swift package version of the library.

Binary change

Total increase: 0 bytes
Total decrease: -1,944 bytes

File	Before	After	Delta
Total	31,053,920 bytes	31,051,976 bytes	🎉 -1,944 bytes

Full breakdown

File	Before	After	Delta
FocusRingView.o	819,504 bytes	819,472 bytes	🎉 -32 bytes
__.SYMDEF	4,823,248 bytes	4,822,544 bytes	🎉 -704 bytes
Avatar.o	601,128 bytes	599,920 bytes	🎉 -1,208 bytes

Verification

All tests pass in both Xcode project and Swift package.

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow