To receive the Demo Tester badge (if you have not received it already), you’ll need to provide proof of ownership.
Post some sort of proof of ownership (e.g. the private message in which you received the key, your product key activation license, your Steam Library, an identifiable video, et cetera…)
The test build itself evolves over time, they’re not separate builds. So I’d really doubt you could play one of the incomplete demos especially when the latest beta by then would likely be a lot better.
Think of it like how you can play 2.0 right now but it’s the latest version 2.2.5 and not one of the older, less complete versions of 2.0.