test: space: minor cleanups

All callsites to stop_stats_pager() should start ignoring the return value,
too.
test: space: test faults on access to kernel range

Or merely outside of the permitted virtual memory range, as for amd64's "top
and bottom" vaddr organization. (obviously untested on amd64.)

Forkserv was altered to report segfaults for access below 64k. This doesn't
break anything else, of course.