DoronD There's 2 quotes in that post, the first one with the big header in it is from the page I originally linked. It literally says and I quote again:
To iterate quickly on the editor, we recommend to set up a test project and open it from the command line after compiling the editor.
Godot editor is built with godot itself, the godot editor UI is built with the UI 'engine' if you will of the godot engine(control type nodes extending the control class), or if by testing the engine UI you mean that you want to modify the editor itself then again you would test it by the same means.
If you mean to ask about unit testing the engine tools build("the editor") that's a whole other thing. You can search for and read about building unit tests for c/c++ I guess, since that's what the engine is primarily programmed in.