This would most likely lack some of the features such as extension support as I’m not 100% sure if that would be possible for Apple Watch. It would most likely be a basic browser with searching functionality, and perhaps video support (not many have this right now although I believe it is possible). It would have access to a search bar, search history and perhaps the ability to access multiple tabs. TestFlight works with Apple Watch but due to the nature of it I believe usually with other apps you have to download the beta on iOS and then it gives you the option to install it on your Apple Watch in the Watch app on your iPhone.