internal/controld: connect to API using ipv4 only

Connecting to API using ipv6 sometimes hang at TLS handshake, using ipv4
only so we can fetch the config more reliably.

Fixed #53

internal/controld/config_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 const utilityURL = "https://api.controld.com/utility"
@@ -24,7 +25,7 @@ func TestFetchResolverConfig(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
 			got, err := FetchResolverConfig(tc.uid)
-			assert.False(t, (err != nil) != tc.wantErr)
+			require.False(t, (err != nil) != tc.wantErr, err)
 			if !tc.wantErr {
 				assert.NotEmpty(t, got.DOH)
 			}