glTF Exporter: Add multimaterial test (#17364)

Follow-up to #17241. 
Tests export of multimaterials / submaterials / submeshes.

<details>
My ulterior motive is to add a glTF exporter test that 1. doesn't use
`File` and 2. doesn't use an existing image as a texture. That way, it
can double as a smoke test for BN.
</details>

Author: alexchuber

packages/tools/tests/test/visualization/ReferenceImages/glTFSerializerMultimaterial.png

@@ -1146,6 +1146,11 @@
             "referenceImage": "glTFSerializerTextureExport.png",
             "errorRatio": 0.1
         },
+        {
+            "title": "GLTF Serializer multimaterial with raw texture",
+            "playgroundId": "#KU72PX",
+            "referenceImage": "glTFSerializerMultimaterial.png"
+        },
         {
             "title": "GLTF Buggy with Draco Mesh Compression",
             "playgroundId": "#JNW207#1",