LookTransformTest : Rationalise test

We can't expect to be able to compute the hashes if the input `look` isn't valid, and were just getting lucky that at present that was possible. Also renamed the function and the `look` value to make it clear what we're testing.

python/GafferImageTest/LookTransformTest.py

@@ -53,7 +53,7 @@ class LookTransformTest( GafferImageTest.ImageTestCase ) :
  groundTruth = GafferImageTest.ImageTestCase.imagesPath() / "checker_ocio_look.exr"
  ocioConfig = GafferImageTest.ImageTestCase.openColorIOPath() / "context.ocio"
 
- def test( self ) :
+ def testInvalidLook( self ) :
 
  n = GafferImage.ImageReader()
  n["fileName"].setValue( self.fileName )
@@ -64,10 +64,10 @@ def test( self ) :
  self.assertImageHashesEqual( n["out"], o["out"] )
  self.assertImagesEqual( n["out"], o["out"] )
 
- o["look"].setValue( "primary" )
+ o["look"].setValue( "nonexistent" )
 
- self.assertRaises( AssertionError, lambda: self.assertImageHashesEqual( n["out"], o["out"] ) )
- self.assertRaises( Exception, lambda: self.assertImagesEqual( n["out"], o["out"] ) )
+ with self.assertRaisesRegex( Gaffer.ProcessException, ".* The specified look, 'nonexistent', cannot be found.*" ) :
+  GafferImage.ImageAlgo.tiles( o["out"] )
 
  def testPrimary( self ):