Is there an Office 365 Post migration checklist or test plan for end users?

I'd say it should be transparent to them, though depending on the migration method you've chosen caveats might apply. I guess access to shared mailboxes/calendars, rooms and so on are one thing to check for, issues with the Name cache and replying to older messages, the occasional broken forwarding or Inbox rule, etc. Again, depends on the method used.

On the bright side, it's a very well known experience now, with people doing migrations to O365 for almost 6 years. Most of the problems are detailed on the internet, so you should generally be OK. And you should definitely also focus on introducing users to the new features they will get!