Is Facebook testing a new Tab Bar UI for its iOS app?

I can confirm this is true. The resources are in the binary.

    iOS_TabBarIcon_Messages@2x.png
    iOS_TabBarIcon_More@2x.png
    iOS_TabBarIcon_NewsFeed@2x.png
    iOS_TabBarIcon_Timeline@2x.png

http://imgur.com/a/pVyiT

There's also a bunch of weird new stuff, like QR codes linking to people's profiles. In Messenger.app there's a disabled function that lets you message people that are physically close to you as well.

I actually prefer tabs rather than a slide out navigation. Then again, Facebook is so complex and has so many features, it would be interesting to see how they would be able to condense things into 4 or 5 tabs.

Highly unlikely. How did you manage to get this screenshot if not made-up in the first place?