refactor: move ErrorWrapperQUICDialer to errorsx (#420)

I needed to add some tests as integration tests due to circular
imports, but this is ~fine because we quite likely want many
integration tests in the errorsx package anyway.

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

internal/engine/netx/netx.go

@@ -171,7 +171,7 @@ func NewQUICDialer(config Config) QUICDialer {
 	var d quicdialer.ContextDialer = &netxlite.QUICDialerQUICGo{
 		QUICListener: ql,
 	}
-	d = quicdialer.ErrorWrapperDialer{Dialer: d}
+	d = &errorsx.ErrorWrapperQUICDialer{Dialer: d}
 	if config.TLSSaver != nil {
 		d = quicdialer.HandshakeSaver{Saver: config.TLSSaver, Dialer: d}
 	}