Probably Ruby. No other language makes testing that easy or fun.

Go does. So does Rust. Both languages have some sort of unit testing facilities packaged with the standard distribution. And both are dead simple to use.

You can't mock out your dependencies as easily in Go as you can monkey patch them in Ruby.

However I've found the explicit error handling in Go to be more valuable in making reliable software than a torrent of test in Ruby, or Java.

The new XCTest in Objective-C is fairly nice as well.