all: use parallel dialer for connecting upstream/api

So we don't have to depend on network stack probing to decide whether
ipv4 or ipv6 will be used.

While at it, also prevent a race report when doing the same parallel
resolving for os resolver, even though this race is harmless.

internal/controld/config_test.go

@@ -9,8 +9,6 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-const utilityURL = "https://api.controld.com/utility"
-
 func TestFetchResolverConfig(t *testing.T) {
 	tests := []struct {
 		name    string