nixos/iftop: add module

This patch is heavily inspired by bd0d8ed8 which added
a setcap wrapper for `mtr` in order to allow running `mtr` without
`sudo`. The need for the capability `cap_net_raw` that can be registered using
`setcap` has been documented in the Arch Wiki: https://wiki.archlinux.org/index.php/Capabilities#iftop

A simple testcase has been added which starts two machines, one with a
setcap wrapper for `iftop`, one without. Both testcases monitor the
bandwidth usage of the machine using the options `-t -s 1` once, the
machine with setcap wrapper is expected to succeed, the `iftop` on the
machine without setcap wrapper is expected to return a non-zero exit
code.