I've found an app called GPS Joystick which uses mock locations and offers an on screen joystick with which you can simulate movement. It costs 0.99$ (0.68€) so I've decided to try it out.
Well, it works. You can give the app any location you want and you can walk around using the joystick. What makes it pretty awkward though, is that the directions you point the joystick to are, obviously, relative to your compass direction and not to the direction your character is facing. This makes controlling the trainer pretty awkward, but it's good enough to test the game.
What I also want to say goes out to all the people who are calling people who want to use things like this cheaters and lazy: I'm not planning on playing this game on a competitive level, I have better things to do than that, I just want to try it out, so don't worry, I'm not ruining other people's fun. In addition, this might not be such an issue for people living in big cities but I'd personally feel very uncomfortable walking around playing Pokemon in my rural German village with about 600 residents where pretty much everybody knows you. I just don't really want people to see me running my 20 year old self running around in our fields and forests staring at my phone. Sure, Pokemon Go might be a huge thing around the world, but here nobody knows anything about these kind of trends. I come from that kind of region where playing video games is still not socially accepted by most older people (and trust me, there are LOADS of old people here), especially when it's a game still considered a children's game.
