[interactive_media_ads] Migrate XCTest to Swift Testing

[jmagman]

Part of flutter/flutter#180787

Test passing:
https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8692723844540973921/+/u/Run_package_tests/native_test/stdout#L16311_1

Adding CHANGELOG override per #10761 (comment)

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[jmagman]

I deleted this convenience method because IMAAdDisplayContainer is still an Objective-C class, and creating @MainActor classes like UIView in a non-MainActor init, I was doing acrobatics with like MainActor.assumeIsolated, but IMACompanionAdSlot is also an Objective-C class, so it wasn't @Sendable, which it needs to be when returned from MainActor.assumeIsolated, etc, etc.

There's some duplication in the tests now, but the one @unchecked Sendable is cleaner than all that.

[jmagman]

Sorry this review is huge, but other than AdDisplayContainerTests.swift it's almost entirely mechanical 1:1.

[gemini-code-assist]

Code Review

This pull request successfully migrates the iOS tests for interactive_media_ads from XCTest to the new Swift Testing framework. The changes are generally well-executed, updating imports, test structures, and assertions. I've provided several suggestions to further improve the tests by consistently using try with throws for test functions that call throwing methods. This will enhance test clarity and provide better failure diagnostics, aligning with modern Swift testing practices.