Comment 1 for bug 517187

If it helps, I believe the config file name gets set in DomainManager.cs in the method "public AppDomain CreateDomain( TestPackage package)". The relevant code snippet is as follows (starts at line 75 in the DomainManager.cs source code for NUnit-2.5.2.9222):

-----snippet start------
configFile = NUnitProject.IsProjectFile(testFile.Name)
    ? Path.GetFileNameWithoutExtension( testFile.Name ) + ".config"
    : testFile.Name + ".config";
-----snippet end------

In the debugger, I see that the ternary operator test returns false so configFile gets set to testFile.Name + ".config".

Since the value of testFile.Name is "nunit.tests.csproj", the value of configFile becomes "nunit.tests.csproj.config".