During a recent project we found a recurring problem where the test view of the unit (& integration) test projects would not load and we could not run tests. This was typically seen by the Test View load progress bar sitting at around 99%. This just adds to the list of "features" of VSTS Unit Test that make life oh so irritating
It is due to the corruption of the *.vsmdi files in the visual studio solution which is caused by auto-merging the content of the file or having the test view property window open when getting the latest version.
The problem is outlined in this blog post (also note the comments for possible workarounds)