I don't think the testing is the main sticking point. The way I see it, if you're clean, why do you give a fuck about what type of test you take? The only thing I can think of is "showing who the boss is! "
What probably happened was that Fury was so concerned with winning the first fight that he didn't really take notice of the financial points of the rematch if he did win.
Now he's probably thinking,
a. I'm the fucking champ now, I should be taking a bigger slice of the cake . And
b. Joshua is earning shit loads of dough, yet I'm the main man, "the man that beat the man!" I should be earning that!
If this is what has happened, then you can understand his frustration, but a contract is a contract, and Fury is risking missing out on the gravy train if he doesn't fight and stay relevant.