I use an inverter and various loads like a hair dryer. Or a dc water pump etc. I find something that develops the load I need. Sometimes I use large wire wound resistors that I find at "specialty" (electronic junk) shops.

I have several shunt operated amp meters to monitor the current and a standard digital meter for the voltage. Shunt meters are easy to make and work well.

The hard part is making sure you are around when the battery reaches the desired "discharged" point. this varies depending on the type of test you want to run. Not hard to make a small arduino (small computer used in robotics) voltage/current monitor that alarms when the set discharge conditions are met.

I can go into it in depth if you are really interested. I live off the grid and do a lot of solar/battery stuff.

What exactly do you want to test??

Nomad