tests: CoreDNSServerAndPorts (#972)

* tests: CoreDNSServerAndPorts

Copy from kubernetes.go and renamed to fit the style, adapted almost
all callers.

This is a mechanicl change, no testdata was changed.

* typos

test/secondary_test.go

@@ -21,15 +21,10 @@ func TestEmptySecondaryZone(t *testing.T) {
 	}
 `
 
-	i, err := CoreDNSServer(corefile)
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
 	if err != nil {
 		t.Fatalf("Could not get CoreDNS serving instance: %s", err)
 	}
-
-	udp, _ := CoreDNSServerPorts(i, 0)
-	if udp == "" {
-		t.Fatal("Could not get UDP listening port")
-	}
 	defer i.Stop()
 
 	log.SetOutput(ioutil.Discard)