Accounting for torsional forces during wing load tests
You would have to calculate the torsional loads for a critical torsional loading case such as full aileron application at limit load, and distribute the sand bags chordwise to produce the calculated spiral loading.  

That's quite an analytical task and is rarely done when the skin thickness/stiffness required for good aerodynamic qualities and flutter resistance is usually quite a bit more than the minimum required for adequate torsional stiffness to resist flight loads. So it kind of takes care of itself anyway and can be covered by analysis with suitable fudge factors, like most of the other load cases on the airframe.