I understand that I’m not allowed to to use 1-Site license on more than one website. Can I use it on a test site before using on my actual website?
Yes, you can use it on a test site, and then actual website. But you have to unpublish your test website with the theme before you publish your actual website with the theme.
Imagine that WEBSITE A is a test site and WEBSITE B is your actual website. You can use a single site license on WEBSITE A but before you can use it without a problem on WEBSITE B, you must either unpublish WEBSITE A (if published with the theme) before publishing WEBSITE B which now uses the theme.
If you don’t want to unpublish WEBSITE A, you can choose a different theme (from weebly gallery) for WEBSITE A before you publish WEBSITE B.