The XCom endpoint joins the XCom and DagRun tables. Currently, the output of the join isn't getting stored as a new `query` variable, as seen in the link to the following PR. We should add a new test to check that these tables are getting joined correctly.

https://github.com/apache/airflow/pull/10594/files/1fe1da978763c9919cee0d2c7eccd96438631993#diff-aa0c2f68c6177fb1587a6de6b5cfb28cR59


I will make these changes after #10594 
tests/api_connexion/endpoints/test_xcom_endpoint.py
dag_id_1 = 'test-dag-id-1'
        task_id_1 = 'test-task-id-1'
        execution_date = '2005-04-02T00:00:00+00:00'
        execution_date_parsed = parse_execution_date(execution_date)
        dag_run_id_1 = DR.generate_run_id(DagRunType.MANUAL, execution_date_parsed)
        self._create_xcom_entries(dag_id_1, dag_run_id_1, execution_date_parsed, task_id_1)
        dag_id_2 = 'test-dag-id-2'
        task_id_2 = 'test-task-id-2'
        dag_run_id_2 = DR.generate_run_id(DagRunType.MANUAL, execution_date_parsed)
        self._create_xcom_entries(dag_id_2, dag_run_id_2, execution_date_parsed, task_id_2)
        response = self.client.get(
            "/api/v1/dags/~/dagRuns/~/taskInstances/~/xcomEntries",
            environ_overrides={'REMOTE_USER': "test"},
        )
        self.assertEqual(200, response.status_code)
        response_data = response.json
        for xcom_entry in response_data['xcom_entries']:
            xcom_entry['timestamp'] = "TIMESTAMP"
        self.assertEqual(
            response.json,
            {
                'xcom_entries': [
                    {
                        'dag_id': dag_id_1,
                        'execution_date': execution_date,