Use for testing

[justincormack]

Once portability is resolved, an in tree netbsd build would be nice, so we can use rumprun for tests. Looking through the tests that use rump (around 90), most of them could be simply replaced by shell scripts that use rumprun, which would also test the userspace tools as well as the syscalls which are currently tested.

In the short term, adding significantly to the rumprun tests and then running rumprun on netbsd head on an increasing range of architectures would be a good start - it is pretty quick writing tests for rumprun.