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
    ...

test/test-execute/exec-privatenetwork-yes.service

@@ -2,6 +2,6 @@
 Description=Test for PrivateNetwork
 
 [Service]
-ExecStart=/bin/sh -x -c '! ip link | grep ": " | grep -v ": lo:"'
+ExecStart=/bin/sh -x -c '! ip link | grep ": " | grep -Ev ": (lo|sit0@.*):"'
 Type=oneshot
 PrivateNetwork=yes