Very briefly document what the co-worker is doing that makes it less efficient for you to have his "help". Have an open conversation with your boss. Show your boss examples of why this isn't working out through what you documented, but try to do it in such a way that it doesn't appear personal or overly critical about the individual, just present the facts. I'm saying this because I had a similar experience myself, and ironically it was doing software testing.
What should happen is your boss should recognize the difficulty you're having due to this individual interrupting you and come up with solutions, possibly the individual going to your boss with questions if your boss has any inkling of the type of work you do, which many of them don't, or find another person to answer this individuals questions, or even realize bringing an unqualified person in just doesn't work out in some cases, and he needs to find other ways to decrease the pressure you're under.
Realize it is our bosses responsibility to find the means for us to be able to do our job properly, and leave it with your boss to deal with. The first step is making your boss aware of what's happening. If you don't talk to your boss your frustration could end up coming out in ways you don't want it to.
Outside of work recognize you have a stressful situation you're dealing with and be diligent in doing all the healthy lifestyle things we need to do to build up our resilience and keep ourselves afloat, including lessening other stressful factors, if possible.
Talk it all out with someone you trust, preferably someone outside of your work. Including us.
Good luck!
