Похоже, это ошибка JetBrains Resharper 9.1 Ultimate Test Runner, ссылающаяся на более старую версию NUnit. Я использую в своей сборке NUnit 3.0.
[Window Title]
Unit Test Runner
[Main Instruction]
Unit Test Runner failed to run tests
[Expanded Information]
at NUnit.Core.Builders.TestAssemblyBuilder.GetCandidateFixtureTypes(Assembly assembly, String ns)
at NUnit.Core.Builders.TestAssemblyBuilder.GetFixtures(Assembly assembly, String ns)
at NUnit.Core.Builders.TestAssemblyBuilder.Build(String assemblyName, Boolean autoSuites)
at NUnit.Core.Builders.TestAssemblyBuilder.Build(String assemblyName, String testName, Boolean autoSuites)
at NUnit.Core.TestSuiteBuilder.Build(TestPackage package)
at NUnit.Core.SimpleTestRunner.Load(TestPackage package)
at NUnit.Core.ProxyTestRunner.Load(TestPackage package)
at NUnit.Core.RemoteTestRunner.Load(TestPackage package)
at JetBrains.ReSharper.UnitTestRunner.nUnit.BuiltInNUnitRunner.RunTests(IRemoteTaskServer server, Dictionary`2 tests, Dictionary`2 fixtures, List`1 explicitly, Boolean useAddins, List`1 assemblies)
РЕДАКТИРОВАТЬ: проблема исчезнет с обновлением Resharper до версии 10.