This is not directly supported right now. The closest thing you could do is:
1. write a plugin to input flight commands.
2. have the plugin track the simulated time.
3. Put the sim in faster-than-realtime.
4. Lower rendering options to get as close to 6x as you can.
The sim does not come in a "headless" variant that bypasses rendering completely.
In x-plane 11 you might find that setting the main monitor to "2-d panel only" gets your fastest possible framerate.