Microsoft Lightweight Automation Framework- Writing your tests in a different web

There is another way to do this. Check this post from the QA team blog._

This is a quick post. In the samples for the framework the test are written in the same system to test.

This is not terrible, since you can easily remove those files for the final compilation, but I prefer to have my test in a different project. Well that’s very easy to do. Just create another web application and follow the same conventions that in the sample to create your test.
Add a reference to the framework, create an App_Code folder if you don’t have one, add a Tests folder inside and create all your test fixtures in it.

Them when loading the pages to test just use a relative path.
The important thing is to run the test runner under the same domain of the app you want to test.