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
@classmethod
    def setUpClass(cls) -> None:
        super().setUpClass()
        with conf_vars({("api", "auth_backend"): "tests.test_utils.remote_user_api_auth_backend"}):
            cls.app = app.create_app(testing=True)  
        create_user(
            cls.app,  
            username="test",
            role_name="Test",
            permissions=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_XCOM),
            ],
        )
        create_user(
            cls.app,  
            username="test_granular_permissions",
            role_name="TestGranularDag",
            permissions=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),