3Delight RendererTest : Fix failure to parse NSI file

The tests affected by this were merged to `1.3_maintenance` where `.nsi` files is written as ASCII. But on main since f417065 we need to use the `.nsia` extension for ASCII, and `.nsi` is binary. This was giving us the following CI failure :

```
Error: ERROR: testUSDLightShaping (IECoreDelightTest.RendererTest.RendererTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/__w/gaffer/gaffer/build/python/IECoreDelightTest/RendererTest.py", line 1316, in testUSDLightShaping
    nsi = self.__parseDict( self.__renderLights( lightSettings ) )
  File "/__w/gaffer/gaffer/build/python/IECoreDelightTest/RendererTest.py", line 1408, in __parseDict
    for i in f.readlines() :
  File "/__w/gaffer/gaffer/build/lib/python3.7/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xcc in position 97: invalid continuation byte
```

python/IECoreDelightTest/RendererTest.py

@@ -1128,7 +1128,7 @@ def __renderLights( self, lightSettings ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		for lightType, position, rotation, geometryType, geometryAttributes, shader, lightParameters, dlParameters in lightSettings :
@@ -1147,7 +1147,7 @@ def __renderLights( self, lightSettings ) :
 		r.render()
 		del r
 
-		return self.temporaryDirectory() / "test.nsi"
+		return self.temporaryDirectory() / "test.nsia"
 
 	def __assertLightSettings( self, nsi, lightSettings ) :