Good testing still helps with distractions because good testing enforces a code style that's very bite-sized; so if your 'tasks' are 20-40m, you can fit a lot of distractions. Also, each task (generally) by virtue of being so small takes up less mental RAM, so you don't have to page out as much when a distraction happens.
Also makes it easy to work "around" the hard part, and come back to it when you know you'll have that uninterrupted time. Or late that night when inspiration strikes and now it's easy :)
