Inject dependency for singleton objective c class in swift test cases

I have one singleton class written in Objective C which is being used in helper class which is also written in Objective C. I am writing a test case for the helper class, but instead of making changes directly to singleton class methods, I want to create a mock singleton class, but I'm not sure how to do that. I don't want to make any changes in helper class either like creating a init method which takes singleton class object and initialise it like mention on other threads for test cases for singleton class.

Thanks for looking the question.