Welcome to EE!
Not knowing your budget or the number of test configurations... Could you consider the following:
- Concrete pad/road with guides installed in most any configuration should work as long as the cart tracks properly perpendicular to the soil.
- Design a retrofit device that can level or compact the soil relative to the cart.
- Place test system in a building and bring soil into building.
- Build wood or other walls and roof around test area to block wind and rain.
Powering the cart with the most readily available power source makes the most sense to me (gas engine, electric or whatever).