touch memory-under-test during init to wire it.
I noticed in taskman that the system's physical memory usage was slowly
creeping upwards as memtest started running, before hitting a steady-state.
This fixes it. I also did some tests to see if the prealloc memory was getting
GC'd before the test ran. Doesn't seem to for me, which is good because if it
was it would have invalidated previous testing as the prealloc stuff wouldn't
have been doing anything.