ArnoldRenderTest : Increase `maxDifference` for image comparison
ArnoldRenderTest : Increase `maxDifference` for image comparison

This should fix sporadic CI failures we've been seeing, and despite being huge is still low enough to guard against the problem the test was intended for.

GafferHQ/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,hradec/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,hradec/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,hradec/gaffer,GafferHQ/gaffer,hradec/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,hradec/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer