You may not realize it, but Visual Studio keeps the runs of your unit tests for a long time, if not forever.
I was cleaning up my hard drive today when I noticed one of my development folders was huge. I turns out I had 300MB and 2,300 files as a result of running my unit tests for my JavaScript converter.
This article describes how to remove those results and recover some disk space:
Unfortunately, it only looks like it reads the last 25 runs ( on Visual Studio 2010 ).
My solution was to simply open Windows Explorer, navigate to the TestResults folder, and delete all of the subfolders within.