Rename an app file extension

In my utilities folder I want to append a letter or number to .app of an app, rendering the app not an app. I am trying to debug a problem with the app, and want to keep the old one, but have it not active, in case the new one continues to be problematic. That was a simple and easy trick with Windows: add a 0 or something to .exe. And it worked every time. Apparently macOS thinks its smarter than the user, and doesn't allow that. No matter what I do to the .app file extension, macOS then appends .app. maintaining the app as an app.

How do I force macOS (10.13.4) to not append .app when I change the .app file extension?