test-network: make kernel send NA with router flag

test-network: make kernel send NA with router flag

If the router interface send NA without router flag, client interface will drop
SLAAC addresses. To make the router interface send NA with router flag,
IPv6 forwarding needs to be enabled.
===
client: NDISC: Received Neighbor Advertisement from fe80::1034:56ff:fe78:9a99: Router=no, Solicited=yes, Override=no
client: NDISC: Invoking callback for 'neighbor' event.
client: Removing NDisc route (configured): dst: 2002:da8:1:99::/64, src: n/a, gw: n/a, prefsrc: n/a, table: main(254), priority: 1024, proto: ra, scope: global, type: unicast, flags: n/a
client: Removing NDisc route (configured): dst: n/a, src: n/a, gw: fe80::1034:56ff:fe78:9a99, prefsrc: n/a, table: main(254), priority: 1024, proto: ra, scope: global, type: unicast, flags: n/a
client: Removing NDisc address (configured): 2002:da8:1:99:1034:56ff:fe78:9a00/64 (valid for 23h 59min 58s, preferred for 3h 59min 58s), flags: manage-temporary-address,no-prefixroute, scope: global
===