How can I test the app in non published theme, so that it won't show up in my live theme?

The best way to test the app in an unpublished theme is to enter your domain where the live theme is published into Seal Subscriptions > Settings > General Settings > Advanced > Disable app on specific urls. This will disable the app on your live theme.
Then, create a subscription rule and use the theme preview URL to see how the app shows up in your non published theme.