ImageReaderTest : Replace deep test image

This one was ancient and upsets the new EXRCore library with the following error :

```
EXR_ERR_MISSING_REQ_ATTR 'chunkCount' attribute for multipart / deep file not found
```

Replaced with a copy of the image loaded and resaved from Gaffer, also downsizing it to 32x32 because nobody needs a 6Mb file just to check we can recognise a deep image.

Author: johnhaddon