[camera_avfoundation] Migrate tests to Swift - part 3 (#8661)

This PR migrates some of Objective-C tests to Swift as a part of Swift migration: [flutter/flutter/issues/119109](flutter/flutter#119109).

I kept the names of test cases the same.

Tests migrated in this PR:
- PhotoCaptureTests
- SampleBufferTests
- StramingTests
- ThreadSafeEventChannelTests

I also migrated two Mock classes to Swift, but I think that for most of mocks it'd be easier to migrate them when migrating the actual implementation to Swift.

packages/camera/camera_avfoundation/example/ios/RunnerTests/CameraTestUtils.h

@@ -30,4 +30,11 @@ extern CMSampleBufferRef FLTCreateTestSampleBuffer(void);
 /// @return a test audio sample buffer.
 extern CMSampleBufferRef FLTCreateTestAudioSampleBuffer(void);
 
+/// Calls `dispatch_queue_set_specific` with a key that is used to identify the queue.
+/// This method is needed for comaptibility of Swift tests with Objective-C code.
+/// In Swift, the API for settinng key-value pairs on a queue is different, so Swift tests
+/// need to call this method to set the key-value pair on the queue in a way that's
+/// compatible withn the existing Objective-C code.
+extern void FLTdispatchQueueSetSpecific(dispatch_queue_t queue, const void *key);
+
 NS_ASSUME_NONNULL_END

packages/camera/camera_avfoundation/example/ios/RunnerTests/CameraTestUtils.m

@@ -127,3 +127,7 @@ CMSampleBufferRef FLTCreateTestAudioSampleBuffer(void) {
   CFRelease(formatDescription);
   return sampleBuffer;
 }
+
+void FLTdispatchQueueSetSpecific(dispatch_queue_t queue, const void *key) {
+  dispatch_queue_set_specific(queue, key, (void *)key, NULL);
+}