USDShaderTest : Work around UsdPreviewSurface token outputs

With the update to USD 24.08, these outputs now being created as StringPlugs. This temporary workaround keeps the tests passing while we consider alternative approaches.

python/GafferUSDTest/USDShaderTest.py

@@ -38,6 +38,8 @@
 
 import imath
 
+import pxr.Usd
+
 import IECore
 import IECoreScene
 
@@ -80,6 +82,7 @@ def __assertShaderLoads( self, name, expectedParameters, expectedOutputs ) :
 
 	def testLoadUsdPreviewSurface( self ) :
 
+		legacyUsdVersion = pxr.Usd.GetVersion()[1] <= 23
 		self.__assertShaderLoads(
 			"UsdPreviewSurface",
 			[
@@ -99,8 +102,8 @@ def testLoadUsdPreviewSurface( self ) :
 				( "occlusion", Gaffer.FloatPlug, 1 ),
 			],
 			[
-				( "surface", Gaffer.Plug, None ),
-				( "displacement", Gaffer.Plug, None ),
+				( "surface", Gaffer.Plug if legacyUsdVersion else Gaffer.StringPlug, None if legacyUsdVersion else "" ),
+				( "displacement", Gaffer.Plug if legacyUsdVersion else Gaffer.StringPlug, None if legacyUsdVersion else "" ),
 			]
 		)