Why do you need to test your own HTML template yourself?
A template that you have created yourself or imported ready-made has not yet been checked by anyone for how it displays. You still need to carry out this check yourself, or have it done by your developer or designer. You may need to adjust the HTML code afterwards.
Just because your newsletter or template displays well in Laposta does not mean it will also be shown properly in all email clients. Every email client has its own quirks. Outlook in particular is known for being difficult.
To test your template, you can send a testmail to your own email addresses. View it in different email clients, in different browsers, and on different screen sizes (responsive design). You can already check what your newsletter looks like before sending it in the preview.
You do not have this drawback if you use one of the templates in Laposta or work with a custom template designed by Laposta. Those templates have already been tested by Laposta for display in various email clients and browsers, on desktop, tablet, and mobile.