She's testing him, yes. There's no real consequence if he fails. But they do need to see if the two of them work. She needs to know if he can truly submit or if he's one of those with the issues letting go, who have to try and assert dominance in all situations. There's nothing wrong with that, but it doesn't interest Natasha.

When he calls her ma'am, when his head bends obediently and without hesitation, she smiles and runs her fingers through his short hair, scratching in the same places that got the most reaction from him the last time. "I'm glad. I don't want to have to retrain a lot of bad habits. You look good on your knees, Dean."