Add test for main dispatch queue processing in CFRunLoop
Add test for main dispatch queue processing in CFRunLoop

This ensures CFRunLoop correctly serves main dispatch queue.

Due to event specifics on Windows there was possibility to break
RunLoop making it think that main queue always has pending
work. The issue is adressed in https://github.com/apple/swift-corelibs-libdispatch/pull/542.

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation