[Tests] Migrate from Quick and Nimble to Swift Testing

[NachoSoto]

Summary

• remove the Quick and Nimble package dependencies
• migrate the remaining specs to Swift Testing
• replace repeated cache and image-view setup with shared fixtures and contract helpers
• express aspect-fit and aspect-fill coverage as parameterized geometry cases
• consolidate the legacy multicast spec into the existing Swift Testing suite while preserving its unique upstream-cache-hit assertion
• replace random test data with deterministic values

Scope

This is an independent test-only PR based on master. It does not depend on the Swift 6/concurrency PR stack and does not change production behavior.

Validation

• xcodebuild -quiet -scheme AsyncImageView -destination 'platform=iOS Simulator,id=E256AEBA-8DCF-4F9C-B2CD-0722FADA3C2F' -enableCodeCoverage NO test
• swiftlint --config .swiftlint.yml --strict
• git diff --check