Arnold tests : Disable problematic tests for Arnold 7 on Mac

These exposed a bug in Arnold 7's render session handling. This could be reproduced with the following simple script :

```
arnold.AiBegin()

while True :

	# Make a universe, do an interactive render.
	# If we don't do this, then all is well.

	if True :

		universe = arnold.AiUniverse()
		arnold.AiSceneLoad( universe, './render.ass', None )
		session = arnold.AiRenderSession( universe )

		arnold.AiRenderBegin( session, arnold.AI_RENDER_MODE_CAMERA )
		arnold.AiRenderEnd( session )

		arnold.AiRenderSessionDestroy( session )
		arnold.AiUniverseDestroy( universe )

	# Make another universe, and load an ass file.
	# Occasionally this will fail, either crashing or
	# not producing any nodes.

	universe = arnold.AiUniverse()
	arnold.AiSceneLoad( universe, './load.ass', None )
	assert( arnold.AiNodeLookUpByName( universe, "test" ) is not None )
	arnold.AiUniverseDestroy( universe )

arnold.AiEnd()
```

We've confirmed that the upcoming Arnold 7.0.0.1 fixes both this simple repro and the original problem in the unit tests. But while that is still pending release, we want to be able to release Gaffer 0.61. Hence we disable the tests for the specific problematic Arnold version.

Author: johnhaddon

python/GafferArnoldTest/ArnoldRenderTest.py

@@ -39,6 +39,7 @@
 import inspect
 import unittest
 import subprocess32 as subprocess
+import sys
 import threading
 
 import arnold
@@ -68,6 +69,17 @@ def setUp( self ) :
 
 		self.__scriptFileName = self.temporaryDirectory() + "/test.gfr"
 
+		if (
+			GafferTest.inCI() and
+			[ int( v ) for v in arnold.AiGetVersion() ] == [ 7, 0, 0, 0 ] and
+			sys.platform == "darwin"
+		) :
+			# Skipping due to multiple-render-session bugs in Arnold 7.0.0.0. We have
+			# reported the bug with a small repro and it is confirmed as fixed for the
+			# upcoming 7.0.0.1 release. It sounds as if the problem exists on Linux too,
+			# but we have only encountered it on Mac so far.
+			self.skipTest( "Exposes Arnold bug" )
+
 	def tearDown( self ) :
 
 		GafferSceneTest.SceneTestCase.tearDown( self )

python/IECoreArnoldTest/RendererTest.py

@@ -37,6 +37,7 @@
 import ctypes
 import json
 import os
+import sys
 import time
 import unittest
 
@@ -53,6 +54,21 @@
 
 class RendererTest( GafferTest.TestCase ) :
 
+	def setUp( self ) :
+
+		GafferTest.TestCase.setUp( self )
+
+		if (
+			GafferTest.inCI() and
+			[ int( v ) for v in arnold.AiGetVersion() ] == [ 7, 0, 0, 0 ] and
+			sys.platform == "darwin"
+		) :
+			# Skipping due to multiple-render-session bugs in Arnold 7.0.0.0. We have
+			# reported the bug with a small repro and it is confirmed as fixed for the
+			# upcoming 7.0.0.1 release. It sounds as if the problem exists on Linux too,
+			# but we have only encountered it on Mac so far.
+			self.skipTest( "Exposes Arnold bug" )
+
 	def assertReferSameNode( self, a, b ):
 		self.assertEqual( arnold.addressof( a.contents ), arnold.addressof( b.contents ) )