Yes, you can. You need to be sure that any configuration you are referencing in your tests actually exist in the app.config of the test project.
In other words, the project where your test is in, does not have a connection string "FertigungRead" defined in its app.config.
One way to do this is to add the app.config of the system under test to the test project as a link, this way any changes happen on both projects.