Vm tests

This is an initial implementation of VM support in the test harness. The approach here is different than the one we discussed offline - instead of having a script produce the appropriate register offsets and memory contents and save them to and envdef file, I had UMR do it directly. I prefer this approach because I think it makes the intent clearer, and thus makes the tests more self-explanatory. It also makes it easier to use the same envdef file for multiple tests.