Let's consider how this test can be implemented using testInIFrame.
First iframe is created automatically by testInIFrame function.
Second iframe have to be created in the onload event handler of first frame.
The test checks must be performed in the onload event handler for the second frame.
This event handler must be wrapped by testInstance.step_func().
However the testInstance is not accessible in tests using testInIFrame.
So there is a choice either make the current test a little bit more complex 
or modify testInIFrame and all tests that are using it.
I have chosen the first option.