Hi George,
For the test to have valid data, in this case, the amount of weight you carry , the weight of your snow shoes should not be a limitation to perform, the limitation should be aerobic capacity and muscle endurance. Snow shoes tend to jam your gait and so ability to perform.
If you decide to do the test with this conditions , the next test should be done the same way in order for you to be able to compare results.
Cheers,
Pedro