Is there some “fake” Flatpak that is suitable for testing purposes? My concern is that using a “real” Flatpak for this could inflate their download statistics.
For what kind of “tests”?
I assume you mean that you want to try out an application on Flathub. But you are concerned that downloading will increase the download count, even if you eventually decide the application is not worth using? And you’d only want to increase the download count if you’d actually use the application long-term?
If I have interpreted your question correctly, the answer is as follows:
No, there is no “fake” Flatpak you could install from Flathub that would not count towards the downloads of the application. Because that is a very specific, and frankly unwarranted concern to have. The download statistics for an application are not really that big of a big deal.
But if you’re really determined to avoid adding to that count, you could download the manifest for the application (a link is available on the app page) and build it on your system and install that. That way, it will not be counted.