Bug 1070845-r1 - [sharedtest] unit test refactoring for new version chai.assert api

What I say is
replace:
```
  spy = sinon.spy(ImageUtils.Downsample, 'sizeNoMoreThan');
```
with
```
  spy = this.sinon.spy(ImageUtils.Downsample, 'sizeNoMoreThan');
```

sorry if this was not clear enough