test-execute: allow sit0@ to exist in private network namespace

It's always visible:

$ sudo modprobe sit
$ sudo unshare -n ip l
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    ...
2: sit0@NONE: <NOARP> mtu 1480 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    ...