[camera_avfoundation] Test utils and mocks swift migration - part 1

[RobertOdrowaz]

Migrates camera test utils and mocks as part of flutter/flutter#119109

Migrates CameraTestUtils and MockCaptureSession to swift. The FLTdispatchQueueSetSpecific function from CameraTestUtils still has to be in ObjC so it's now separately in QueueTestUtils.

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[RobertOdrowaz]

FYI I've renamed CameraTestsUtils back to CameraTestUtils as I didn't intend to change the name before it was just a typo

[hellohuanlin]

one nit and LGTM