Adds testing for IPv6.

Currently IPv4 is hardcoded, meaning IPv6 should not work even for
the loopback interface.

scripts/runtests.sh

@@ -250,20 +250,24 @@ if [ "${RELEASE%.*}" -ge 20 ]; then
     # Assert that only loopback interface is accepted.
     interfaces=($(ip --json link show up | jq -r '.[]["ifname"] | select ( . != null )'))
     for if in ${interfaces[@]}; do
+        for ipv in 4 6; do
             curl_ec=0
-        timeout 10s curl "http://localhost:$port/meta/status" --interface $if || curl_ec=$?
-        # Loopback should exit 0
+            timeout 10s \
+                curl -$ipv "http://localhost:$port/meta/status" --interface $if \
+                || curl_ec=$?
+            # Loopback should exit 0 on IPv4
             if [ $if = "lo" ]; then
-            if [ $curl_ec -ne 0 ]; then
+                if [ $curl_ec -ne 0 -a $ipv -eq 4 ]; then
                     loopback_failed=1
                 fi
-        # Other interfaces shoud not
+            # Everything else should not.
             else
                 if [ $curl_ec -eq 0 ]; then
                     unallowed_failed=1
                 fi
             fi
         done
+    done
     kill $subiquity_pid
     if [ $loopback_failed -ne 0 ]; then
         echo "Loopback was expected to connect"