I don't know of anyway to do this in natively in VS but you should look into a tool like NCrunch, a continuous test runner, which will run your tests not only when you build your 'normal' project but also when you make any change to the project.
There are other continuous test runners out there, like MightyMoose, the source of which is here I believe
I'm in no way affiliated with NCrunch (or mighty moose), but I think its awesome and can't imagine life without it now.
Its not free though so whether its useful for you I can't say.