[test][assetwidget] Improve algorithm migration test

It ensure that the declaration loaded matches the one
from the migrated algorithm.

beat/editor/test/test_assetwidget.py

@@ -23,6 +23,7 @@
 #                                                                             #
 ###############################################################################
 
+import json
 import os
 import random
 
@@ -591,6 +592,10 @@ class TestAssetWidget:
             asset_widget.loadAsset(asset)
 
         assert asset_widget.current_asset.name == "v1/sum/2"
+        assert (
+            json.loads(asset_widget.json_widget.toPlainText())
+            == asset_widget.current_asset.declaration
+        )
 
     def test_experiment_error_hinting(self, qtbot, test_prefix, beat_context):
         asset_widget = AssetWidget()