There're two ways to do this in Product Fruits. Choose the one that suits your situation best.
Create a segment (let's call it Testers) consisting of users who are gonna be testing your onboarding flows. You can list each email individually or you can segment them by entering the email domain.
Then you restrict the content to be shown to the Testers segment only. For example in the tours editor, choose "Display for" and select the Testers segment.
When your tours are ready to be released to everyone, change the targeting to All users. Or create a new custom segment (e.g. Users on mobile).
Create two workspaces - Staging and Production.
Build tours on the staging workspace, where it will be tested & debugged. When you're happy with the result, copy the content to your production workspace.