Hi Ian,
I've been in your shoes before. At one point I was sending actual email campaigns to myself as a "test" because I didn't know how. The steps to send a test email via the new email editor are below:
1. Go into the email draft
2. Click on the "Simulate Content" button at the top.

3. From here, you'll add yourself as a person on the left side of the screen, and click the "send proof" button on the top-right.
4. Input the email you want to send the proof to and select who you want to simulate as in the drop-down menu. You can then finally click the blue "Send Proof" button in the top right of your screen.
I hope that helps!