Try app demo in your own account before making it live

Description of request:
Allow developers to try the app demo experience from end-to-end in the developer center, before releasing it on their app.

Current blocker:
Today, developers cannot try the app demo experience. They must be in the app marketplace, and must release their demo to the public if they want to try it.

What it would improve:
Give developers more confidence in their demo experience and be able to build a more user-friendly flow for potential customers.

NOTE: I opened this feature request from @lubos.husivarga’s post here:

It would be great to have this magic ‘Start Demo’ button in our developer section to easily check our updates to the demo workspace, is it feasible?

To add to this, the ability to test different roles will be of great value too.