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/netxmocks/dialer.go

@@ -15,7 +15,7 @@ type Dialer struct {
 	MockDialContext func(ctx context.Context, network, address string) (net.Conn, error)
 }
 
-// DialContext implements Dialer.DialContext.
+// DialContext calls MockDialContext.
 func (d *Dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
 	return d.MockDialContext(ctx, network, address)
 }