When running your unit tests with ReSharper, apparently your assemblies all get copied to a temporary directory, where your project gets build and in which the actual tests are run. This allows you to start a second build of a project, while your tests are still running. Handy as this might be, it does give problems sometimes. If, for instance, some of your tests rely on additional files to be present in your output directory (like resource files), they will not be found, since they are not being copied to the temp directory in which the ReSharper tests run.
Only option is, to turn off the shadow-copy capability in the ReSharper Options window. Now your tests will be run in your actual output directory.
I ran into this issue with classes that utilize Assembly.GetExecutingAssembly to get the full file path to a resource. While my test cases ran fine while using the MSpec runner (which uses a rake script which copies all necessary files), they failed using the NUnit runner utilized by ReSharper. Turning of the shadow-copy option resolved the problem.