This is the tutorial, and the tutorial is written with WebAppCreator and Ant, or Eclipse, in mind.
It's probably a good idea to update it to "something else" (Maven or Gradle; I'd personally prefer Gradle), but that's much broader than just JUnit.
The doc for testing is at
http://www.gwtproject.org/doc/latest/DevGuideTesting.html, and should probably either be updated to also cover Maven (that webAppCreator also supports, but then make sure you use GWT 2.8, as things have changed between 2.7 and 2.8), or to remove everything specific to build tools.
If it doesn't take too much time making the changes, feel free to make pull requests. If you think it'd be quite long, then better wait for feedback from others ;-)