Add assertion message if testDns ipv6 lookup reutrns ipv4.

Some partner test networks connect by VPN and if the connection
fails during the testing this test will fail as the local internet
provider intercepts the DNS request and returns an ipv4 address.

Added a message to check the network configuration and rerun the test.

Bug: 26560000
Change-Id: I54e4976b2cef549bfe9fd55ff9609ba2e6513239

tests/cts/net/src/android/net/cts/DnsTest.java

@@ -96,7 +96,8 @@ public class DnsTest extends AndroidTestCase {
             if (DBG) Log.e(TAG, "ipv6.google.com gave " + addr.toString());
         }
 
-        assertTrue(foundV4 == false);
+        assertTrue("[RERUN] ipv6.google.com returned an ipv4 address, check your network's DNS connection.",
+            foundV4 == false);
         assertTrue(foundV6 == true);
 
         assertTrue(testNativeDns());