Added a failing test case to GafferImageTest.ImageTransformTest

It would appear that in the last 8 months a bug has been introduced into the ImageTransform node which is corrupting it's output. This test compares the current result against that of a previous version (0.83.0).

I am not sure what is causing the bug yet but it looks like a cache issue. In the mean time, this failing test case should highlight the issue.