nUnit tests looking for {assembly-name}.dll.config
The solution for this is to use the console switch /domain=multiple. When you change the default domain usage setting to use a separate AppDomain per Assembly it causes NUnit to load the projects app.config. This option is only available in NUnit 2.5 or later.
nunit-console your.project.csproj /domain=multiple
Not sure if this still applies to the OP's question , but when you build the test project multiple .dll.config files are copied over . If you need to have custom configuration for the tests , copy the .config into the test project . Then in the Properties pane set Copy to Output Directory to Copy Always , this will overwrite the projects config file .
The quick answer is that NUnit doesn't manage config files. It leaves the handling of configuration files to the framework:
If you want NUnit to use a specific configuration file, you can create an NUnit project, and copy the configuration file you wish to use into [NUnitProjectName].config. This configuration file would need to reside in the same directory as your NUnit project.