Swift Unit Test how to set with a private setter

Question

Here is a little background:

I am trying to test some functions. I CANNOT change anything in the function or any class variables.

So I am calling this function in my unit test, the problem is that, there is a boolean flag, this method simply will just not execute if this flag is not set to true. But this flag has a private setter, so I can directly set it.

I asked around someone told me to use a extension IN the test file to set the variable, but I've tried I don't think I can set it if the extension is not in the same file as the function. Am I doing something wrong? Or is there any other way to test it?