Fix flaky test_get_context_in_old_style_context_task test (apache#28585)

The test was flaky because it was passing schedule "@once" as
default_args (wrongly) - as the result this test was set to
the default "daily" and since start date was set to 1st Jan 2022
it executed longer and longer every day (running the Python
Operator for every day between 1st Jan and now.

This PR cahnges the test to correctly pass "@once" schedule via the
`schedule' parameter.

tests/operators/test_python.py

@@ -924,21 +924,9 @@ def test_virtualenv_serializable_context_fields(self, create_task_instance):
             *PythonVirtualenvOperator.AIRFLOW_SERIALIZABLE_CONTEXT_KEYS,
             *intentionally_excluded_context_keys,
         }
-
         assert set(context) == declared_keys
 
 
-DEFAULT_ARGS = {
-    "owner": "test",
-    "depends_on_past": True,
-    "start_date": timezone.datetime(2022, 1, 1),
-    "end_date": datetime.today(),
-    "schedule_interval": "@once",
-    "retries": 1,
-    "retry_delay": timedelta(minutes=1),
-}
-
-
 class TestCurrentContext:
     def test_current_context_no_context_raise(self):
         with pytest.raises(AirflowException):
@@ -1000,14 +988,24 @@ def clear_db():
     clear_db_runs()
 
 
+DEFAULT_ARGS = {
+    "owner": "test",
+    "depends_on_past": True,
+    "start_date": datetime(2022, 1, 1),
+    "end_date": datetime.today(),
+    "retries": 1,
+    "retry_delay": timedelta(minutes=1),
+}
+
+
 @pytest.mark.usefixtures("clear_db")
 class TestCurrentContextRuntime:
     def test_context_in_task(self):
-        with DAG(dag_id="assert_context_dag", default_args=DEFAULT_ARGS):
+        with DAG(dag_id="assert_context_dag", default_args=DEFAULT_ARGS, schedule="@once"):
             op = MyContextAssertOperator(task_id="assert_context")
             op.run(ignore_first_depends_on_past=True, ignore_ti_state=True)
 
     def test_get_context_in_old_style_context_task(self):
-        with DAG(dag_id="edge_case_context_dag", default_args=DEFAULT_ARGS):
+        with DAG(dag_id="edge_case_context_dag", default_args=DEFAULT_ARGS, schedule="@once"):
             op = PythonOperator(python_callable=get_all_the_context, task_id="get_all_the_context")
             op.run(ignore_first_depends_on_past=True, ignore_ti_state=True)