refactor: Unify all json.(loads|dumps) usage to utils.json (#28702)

Co-authored-by: Eyal Ezer <eyal.ezer@ge.com>

tests/integration_tests/base_tests.py

@@ -19,7 +19,6 @@
 
 from datetime import datetime
 import imp
-import json
 from contextlib import contextmanager
 from typing import Any, Union, Optional
 from unittest.mock import Mock, patch, MagicMock
@@ -43,6 +42,7 @@ from superset.models.slice import Slice
 from superset.models.core import Database
 from superset.models.dashboard import Dashboard
 from superset.utils.core import get_example_default_schema, shortid
+from superset.utils import json
 from superset.utils.database import get_example_database
 from superset.views.base_api import BaseSupersetModelRestApi