What technology or techniques are used to test rovers (that are designed to work on the moon, for example) on earth ?
How can the environment (e.g. gravity) of the moon be simulated on earth?
One cannot simulate microgravity on Earth, except for a few seconds at a time in a diving airplane. One should be able to simulate low gravity such as the 0.17g lunar gravity for a few more seconds. The simulated (low and?) microgravity business seems to be expanding with suborbital ambitions and Swiss Space Systems.
But I don't think it would be practical for much rover testing. And I think that the engineering is based on calculations without any actual tests aboard diving airplanes. (If real lunar gravity testing was done for the Apollo program I'd love to hear about it).
NASA built the Reduced Gravity Simulator, which allowed astronauts to train for walking on the Moon:
I haven't been able to find a similar system for a rover.
For earth-bound tests where gravity is important (e.g. for checking the suspension), NASA often builds a replica vehicle at a lower weight (for Mars with a gravity of ~0.3G the replica will weigh 0.3 times as much as the original rover). Here's a replica of Curiosity being tested on Earth:
As you can see, the suspension looks like Curiosity's while the body is much smaller to get the weight down.
Other test vehicles are built to test other aspects (thermal and vacuum tests, for instance), each test vehicle is built to simulate one aspect of the rover.
For a robotics competition I attended, the whole robotics challenge (with mobile robots) was on a slick surface, and the robots had to have slick wheels. The idea was that the friction could be lowered to that of something on the lunar surface, but by altering the coefficient of friction, not the normal force.
Parts of the rover, such as the suspension, drive train, and autonomous decision making, navigation, and obstacle avoidance capabilities can tested by creating a multi-physics model of the rover and testing it digitally in a simulated environment. The Mars Desert Research Station VR is an example of such an environment (although made for Mars as opposed to The Moon). Because the environment is simulated, the physics engine can be programmed to simulate lunar gravity.
The rover can also be tested in a laboratory environment with lunar regolith simulant to see test wheel traction, wear, and the moving parts' resilience to fouling. The laboratory environment may be evacuated to ensure the simulated regolith behaves the way it would on the moon. The environment can also be heated and cooled to resemble the temperature swings on the moon.