Unit Testing with NUnit and Configuration Files
"When you are using NUnit and configuration files, the name of the configuration file must be the same as the name of the assembly with the .config suffix appended. For example, the assembly that contains the tests is named DataAccessLayer.dll. The name of the configuration file must be named DataAccessLayer.dll.config and it must be placed in the same directory as the assembly itself for the NUnit executable to find it."
- Test-Driven Development in Microsoft .NET