refactor(netxlite): make sure we always use netxmocks (#399)

* refactor(netxlite): make sure we always use netmocks

While there, improve logging and make sure we test 100% of the
package with unit tests only. (We don't need to have integration
testing in this package because it's fairly simple/obvious.)

Part of https://github.com/ooni/probe/issues/1505

* further improve logs

internal/netxlite/resolver.go

@@ -48,9 +48,13 @@ func (r ResolverLogger) LookupHost(ctx context.Context, hostname string) ([]stri
 	r.Logger.Debugf("resolve %s...", hostname)
 	start := time.Now()
 	addrs, err := r.Resolver.LookupHost(ctx, hostname)
-	stop := time.Now()
-	r.Logger.Debugf("resolve %s... (%+v, %+v) in %s", hostname, addrs, err, stop.Sub(start))
-	return addrs, err
+	elapsed := time.Since(start)
+	if err != nil {
+		r.Logger.Debugf("resolve %s... %s in %s", hostname, err, elapsed)
+		return nil, err
+	}
+	r.Logger.Debugf("resolve %s... %+v in %s", hostname, addrs, elapsed)
+	return addrs, nil
 }
 
 type resolverNetworker interface {