When testing a tank, it is important that it be on a flat reasonably level surface. Setting it up haphazardly could cause a leak or even a failure. That is to say, if you don't set it up with the same care as you would when setting up inside you can actually put stresses on the tank, that would cause a perfectly good tank to break.
Your idea of placing 2 x 4s on the ground, would work if the pieces of wood could be placed in exactly the same plane, which would be difficult, especially with 3. A lawn would probably be the last place I would try something like this, and would consider myself a bit of a professional, with regards to this type of work, as a millwright. I would look for a place that is more substantial such as a concrete slab, porch or deck.
Join a fish club. Don't have one in your area? Start one.