[local_auth_darwin] Migrate XCTest to Swift Testing

[jmagman]

Part of flutter/flutter#180787

Adding CHANGELOG override per #10761 (comment)

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
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