If you try to debug a MS Test unit test in VS 2008 but stop the debugger before the test is completed (abort the test) the test status will be marked with "Aborted". Which is fine and expected. But if you then try to execute any other test, whether this is debugging a test or just executing a test, it will fail and the status of all attempted tests will be "Not Executed". And from here in no tests will ever execute again..until you kill process "vsperfmon.exe". If you kill that process, then unit tests will then continue to execute.
This defect seems to be fixed in VS 2010 which is great but it still exists in VS 2008 which is a pain as Windows Mobile developers still require VS 2008 to develop WM apps (this defect applies to both desktop and device tests).
There is a bug report on Microsoft Connect: https://connect.microsoft.com/VisualStudio/feedback/details/299925/after-debugging-a-unit-test-vsperfmon-exe-must-be-killed-to-be-able-run-further-tests
As always, if you find this defect annoying, then use the site above to vote for a fix.
This bug has been around for sometime and it's only today I learned the workaround!