Modify trace api

@mortenvp I finished the unit tests and fixed the todos as good as i could. Please let me know what you think? :)
No you would have to store the result in a boolean and pass that to EXPECT_TRUE, but I don't think that was your point :)
I did it like this so that you get to see how the strings differ, so instead of seeing that false is not true you get to see that "some output" != "some other output", if that makes sense?