tests: fix test_550_ipv6_spoof_ip

Use 'ip route replace' instead of 'ip route add' to not fail on already
existing route entry. This worked before only because of a race
condition with a network setup.

qubes/tests/integ/network_ipv6.py

@@ -345,7 +345,7 @@ class VmIPv6NetworkingMixin(VmNetworkingMixin):
         self.loop.run_until_complete(self.testvm1.run_for_stdio(
             'ip -6 addr flush dev eth0 && '
             'ip -6 addr add {}/128 dev eth0 && '
-            'ip -6 route add default via {} dev eth0'.format(
+            'ip -6 route replace default via {} dev eth0'.format(
                 str(self.testvm1.visible_ip6) + '1',
                 str(self.testvm1.visible_gateway6)),
             user='root'))