Ensure test-resources folder always exist before running tests

Solves issue #60 
I dont' understand. Here is what I got in my build now : 

```bash
:app:processTestDebugResources UP-TO-DATE
:app:copyDebugTestResources
```

and it works fine. I think the processTestResourceTask is the android task that normally processes all resources, creates a merge of variant, test and filter stuff etc.. 
THEN we need to copy that to test-resources folder for the variant.

So to my mind this new order is correct.