Any Alternative for Test, Dev, UAT for SharePoint Online to test solution before taking to live

Dear All,

• We are having huge Content DB of SharePoint 2013 and we convinced the features of SharePoint Online and it's advantages and our client had given the signal to move all the solutions for SharePoint Online.
• As we all knew for SharePoint On-premise solutions we do have Dev, TEST , UAT Environments were we implement and validate the requirements before we take ahead to production
• But how do we test our solution on SharePoint Online including permissions and all other features(Permissions).