I don't know that the conditions on the Moon are close enough to the conditions on Mars to justify lunar testing. Antarctica can be dangerous, and the Amazon can be dangerous, but it isn't really safe to say that if you can survive the rigors of one, you can survive the other.

Antarctica provides an easily accesible testing location similar to Mars, and we don't have to squander any expensive launches on setting up lunar training grounds. If you want a place to test the spacecraft in vacuum, we could launch a mission into a high Earth orbit. That way we have actual travel condition tests, and if there's a problem, then Earth isn't nearly so far away. Of course, you couldn't do the same science in Earth orbit that you'd do on the Moon, but if you're testing how the crew will react to several months in a confined space, you won't be letting them outside to explore anyway.

On the other hand, if (say) the US elected a President that was clearly dedicated to manned space exploration, the Moon might be an ideal starting-off point. A return to the Moon might be easier for Congress and the citizenry to swallow, and once we have the hardware to make it to the Moon, we could push farther and say "well, now that we have hardware to make it to the Moon, we have Mars within our reach. Let's go."