tools/lib/testutil: make godoc server listen on localhost only.
tools/lib/testutil: make godoc server listen on localhost only.

I was notified by the security team: https://buganizer.corp.google.com/issue?id=19455535

After this change, I plan to put godoc server behind corpsso (as Jenkins),
and show a generic 404 page when a nonexistent page is visited.

Change-Id: I08fa68162856d2b2a9ec594b4837fd4f89bd160b

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools