C# - reading an app.config in MSTest / Unit tests

Question

I'm trying to work out how to (and at the same time best practice) use an app.config file in unit tests (MSTest specifically).

So I have the actual project then a separate project. the actual project has an app.config - and it seems that the unit tests are using that file and not ones I am trying to specify.

I have tried the following:

1. Putting a copy of app.config in the root of the test project, removing some key config sections that are used within the code so thus should cause exceptions and failures - this doesn't seem to work

2. using

   [DeploymentItem("app.config")]
   

Which I believe picks up the app.config from the test project root and deploys that?? Strangely, I also tried to change the path to

[DeploymentItem("Data\\app.config")]

Which is a directory I use to load other deployment items - however it complains that it cannot find the app.config.

What am I doing wrong here - and also am I doing it the right way (assuming I am missing something simple). I basically want a separate app.config where I can use specific test settings rather than use the real app.config copied or referenced.