PositiveGpuAVDescriptorPostProcess.SharedDescriptorDifferentOpVariableId sporadically asserts

The test `PositiveGpuAVDescriptorPostProcess.SharedDescriptorDifferentOpVariableId` will sporadically assert:

```
[ RUN      ] PositiveGpuAVDescriptorPostProcess.SharedDescriptorDifferentOpVariableId
Assertion failed: false, file C:\Projects\Vulkan-ValidationLayers\layers\gpuav\instrumentation\post_process_descriptor_indexing.cpp, line 251
```

Seems to happen about 1 out of every 10 runs. 