[file_selector] Switch to Swift Testing

[stuartmorgan-g]

Updates the iOS and macOS Swift unit tests to use Swift Testing rather than XCTest.

Part of flutter/flutter#180787

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request migrates the iOS and macOS example runner tests for the file selector plugins from XCTest to the Swift Testing framework, replacing test classes, assertions, and expectations with @Suite, @Test, #expect, and confirmation. The review feedback recommends utilizing try #require(...) to safely unwrap optionals instead of force-unwrapping or using conditional binding, and simplifying array assertions to avoid potential out-of-bounds errors.