Many of these "solved problems" have also been solved with the other languages, albeit not always at the surface level. For Node specifically:
WebSockets -> SockJS, Socket/Engine.io
Concurrency -> Generator Functions
Testing -> import assert from 'assert' and npm test
Syntax -> Some great improvements with ES6 and solid styleguides (airbnb, standard) + eslint as an autoformatter
The major win here for Go is that as a new platform it was designed with simplicity in mind. These features are probably easier to pick up in Go than any of the alternatives, which is great. However, don't make the mistake of thinking that Go is the first language to solve these problems.