[WIP] device: run testns.sh from a Go test harness

[WIP] device: run testns.sh from a Go test harness

objectives:
- to make it easy to run these tests during go development
  (TODO: running sudo go test is weird, is there some
  alternative? does docker provide a namespace hole for
  normal users if it's installed?)
- to make it easy to run some part of the script, e.g.
  just run the sticky sockets test without waiting on
  the prior iperf tests

not clear yet if the shell-inside-go is worth it

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>