[video_player] Convert iOS unit tests to Swift

[stuartmorgan-g]

Converts all of the native unit tests in video_player_avfoundation to Swift:

• For the UI integration tests, it's just a direct Swift conversion.
• For the unit tests, I first did a direct conversion to Swift, including ensuring that everything passed in that version, then converted from XCTest to Swift Testing.

To minimize churn, I deliberately didn't address other technical debt in the tests (e.g., testing of player instances by making an entire plugin instance just to make a player). Some changes were necessary though:

• Added some missing NS_ASSUME_NONNULL_BEGIN annotations to headers, to avoid having to deal with optional values in the Swift version of tests that couldn't ever actually be null.
• Changed a utility method to take the necessary parts of AVAssetTrack instead of AVAssetTrack. The tests for that worked by subclassing AVAssetTrack even though it has no public initializers, which was extremely sketchy. It worked okay in Obj-C because Obj-C will let you get away with almost anything, but it doesn't compile in Swift.
• Some tests that used Expectations to watch for KVO on player items extracted from the player internals have instead been changed to watch for player-level observable events. This is because I was having a lot of trouble getting KVO of AVPlayerItem to actually work in Swift, and rather than try to fix that when it wasn't great test design anyway, I just changed to using something that wasn't an internal detail anyway.

Part of flutter/flutter#119105

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• I linked to at least one issue that this PR fixes in the description above.
• 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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• 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.

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[stuartmorgan-g]

It may be useful for review to look at the changes before ec4b403 first, to see the more direct conversion, and then review ec4b403 separately to see what I had to change for Swift Testing.

[hellohuanlin]

nit: separate lines per style guide -> "Line-Wrapping" section.

Filed: flutter/flutter#182175

[hellohuanlin]

nit: final class (here and rest of this file)

[hellohuanlin]

Interesting. AVPlayer is main actor, but seek is explicitly marked as non-isolated, meaning that Apple likely has some internal synchronization to make seek safe to be called from different threads.

We could make these 3 thread-safe too, but since it's testing only and we don't access it from different threads, using nonisolated(unsafe) is fine here

[hellohuanlin]

Having continuation as ivar is non-standard practice, as it's very easy to misuse (e.g. call resume twice). How about:

class StubEventListener... {
  var onInitialized: (() -> Void)?
  ...
}

@Test func fooTest() {
    // ...
    await withCheckedContinuation { continuation in
      listener.onInitialized = { continuation.resume() }
    }
  }

[hellohuanlin]

nit: private static let

[hellohuanlin]

Swift auto-generate the async/await API based on completion API. We should already be able to call

let error = await player.seek(to: time)

[hellohuanlin]

This and other places in this file: if videoPlayerDidInitialize is not called, the test suite would get stuck (rather than marking this test as failure and continuing).

SwiftTesting has a time limit trait: https://developer.apple.com/documentation/testing/trait/timelimit(_:), but it's iOS 16+ only.

Google search sends me to https://www.donnywals.com/implementing-task-timeout-with-swift-concurrency. I cleaned up the code and updated it for Swift 6:

enum TestError: Error {
  case timeout
}

func withTimeout(seconds: TimeInterval, operation: @escaping @Sendable () async throws -> Void) async throws {
  try await withThrowingTaskGroup(of: Void.self) { group in
    group.addTask { try await operation() }
    group.addTask {
      try await Task.sleep(nanoseconds: UInt64(seconds * 1_000_000_000))
      throw TestError.timeout
    }
    
    defer { group.cancelAll() }
    try await group.next()!
  }
}

Then in the test:

    try await withTimeout(seconds: 5) {
      await withCheckedContinuation { continuation in
        listener.onInitialized {
          continuation.resume()
        }
      }
    }

I verified it working by adding a sleep:

    try await withTimeout(seconds: 5) {
      try await Task.sleep(nanoseconds: 10 * 1_000_000_000)
      // ...
    }