Tests: USD: Run the compliance validator for the MaterialX export test

We recently pulled in the upstream patch to address the incorrect
validation error we were experiencing. This was the only test which
previously required the validator to be disabled.

Pull Request: https://projects.blender.org/blender/blender/pulls/138289

tests/python/bl_usd_export_test.py

@@ -1118,15 +1118,14 @@ class USDExportTest(AbstractUSDTest):
         export_path = self.tempdir / "materialx.usda"
 
         # USD currently has an issue where embedded MaterialX graphs cause validation to fail.
-        # Skip validation and just run a regular export until this is fixed.
+        # Note: We use the below patch for now; keep this in mind if it causes issues in the future.
         # See: https://github.com/PixarAnimationStudios/OpenUSD/pull/3243
-        res = bpy.ops.wm.usd_export(
+        res = self.export_and_validate(
             filepath=str(export_path),
             export_materials=True,
             generate_materialx_network=True,
             evaluation_mode="RENDER",
         )
-        self.assertEqual({'FINISHED'}, res, f"Unable to export to {export_path}")
 
         stage = Usd.Stage.Open(str(export_path))
         material_prim = stage.GetPrimAtPath("/root/_materials/Material")