I would just get a cheap slick tyre?  You can get specific tyres for turbo trainers (I think they are more slick so last longer), but dunno if you would get them to fit a mtb wheel.  I just use normal tyres on mine as I can't be bothered to have multiple wheels or to change the tyre and it works fine (on road bike with road tyres though)

problem with mtb tyres on the turbo is that they make a lot of noise, and i think they would probably wear out quickly.

Semi slick would be better, and would be fine on towpaths and fireroads outside, particularly if you leave a normal mtb one on the front

or you can get a spare wheel, but obviously this is a bit pricey, and also means two cassettes, and then if you ride one wheel loads more than the other one cassette could get more worn which would be very annoying!