I am going to be doing an 'infomercial' type production in the next few weeks and I need to do some testimonial type interviews outside. What would work best for audio?
This is what I have to work with.
- AT-897 Shotgun Microphone
- Sennheiser Evo-100 Wireless lav kit

Would the best bet be to wire up the speaker with the wireless and also run the AT-897 on a boom or stand?

Yes, I would run both and make sure you have adequate wind protection for each mic. I would try to keep the 897 at 24 to 30 inches from the subject. As you get further away you'll quickly lose the ability to use this track as a replacement if the primary lav track has a problem. And I don't mean trying to splice together a fix for a dropout or a hand hit on the lav, that would be nearly impossible to make sound correct. I mean as a total replacement for a particularly good answer that had a problem at some point. You can intercut different interviewee's if you have one cut from a particular subject that sounds different.
This assumes you won't need the interviewer's questions, which is good practice for testimonials. The answer should be able to stand totally on its own so I would dedicate both mics to the subject.

What Jay said... ditto. That's exactly the way I handle it, and yes, I've had to swap tracks because of excessive wind, ambient or other noise.