How to secure connection strings used by Unit Test in TFS?

I have a project with unit tests in TFS 2010. My unit test need to connect to database, so it has to keep connection string somewhere. This connection string is not needed for main application, it is only used by Unit Test project. I am currently storing this connection string in app.config file of unit test project. So, any developer with access to source code can view database login and password, which is not good for security. Is there a way to store this connection string somewhere in TFS, so that it is not exposed to everyone, but unit test project can still use it?