Instead of a doing it in a post build event, that will happen every time you compile, I would look at setting up a Continuous Integration Server like CruiseControl.Net. It'll provide you a tight feedback cycle, but not block your work with running tests every time you build your application.
If you are wanting to run the set of tests you are currently developing, Anton's suggestion of using ReSharper will work great. You can create a subset of tests to execute when you wish and it's smart enough to compile for you if it needs to. While you're there picking up the demo, if you don't already have a license, pick up Team City. It is another CI server that has some promise.
If you are wanting to use this method to control the build quality, you'll probably find that as the number of tests grow, you no longer want to wait for 1000 tests to run each time you press F5 to test a change.