ArnoldRenderTest : Relax test threshold
ArnoldRenderTest : Relax test threshold

This should hopefully deal with some false failures we're seeing on CI. As documented in the comment above, although the allowed delta is large, it still guards against the problem it was intended to.

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