In the current version there is no dedicated send test email button. What you can do instead:

• Use Preview (available in the Content step, the Review step, the template designer and on the campaign page) to open the rendered email in a new tab.
• Create a List containing only your own email address (Marketing > Email > Lists > Add List, then Add Contact to add yourself to it), send a copy of the campaign to that list, review it in your inbox, then create the real campaign for your full audience. Remember each test counts toward your monthly email usage.
• For flows, the Design email view in the block editor also has a Preview.