Could not load file or assembly 'Cesium.Runtime' in tests

Occasionally Codegen tests in Visual Studio (at least where I work) fails with following exception

```
System.IO.FileNotFoundException : Could not load file or assembly 'Cesium.Runtime, Version=0.0.1.0, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.
CodeGenTestBase.CreateAssembly(TargetRuntimeDescriptor targetRuntime, TargetArchitectureSet targetArchitectureSet, String namespace, String globalTypeFqn, String[] referencePaths)
CodeGenTestBase.GenerateAssembly(String[] sources, TargetRuntimeDescriptor runtime, TargetArchitectureSet arch, String namespace, String globalTypeFqn, String[] referencePaths) line 47
CodeGenTestBase.GenerateAssembly(TargetRuntimeDescriptor runtime, TargetArchitectureSet arch, String namespace, String globalTypeFqn, String[] sources) line 35
CodeGenTypeTests.DoTest(String source, String namespace, String globalTypeFqn) line 12
CodeGenTypeTests.StructForwardDeclaration() line 415
RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
```