Remove executor error unit test

This unit test was designed to trap unusual errors when setting up an
Executor and therefore relied on being able to create just such an
error. The previous version relied on the fact that dag_ids are fragile
but moving to deferred dag assignment fixed that fragility and “broke”
the unit test. The only other solution I’ve found so far is to take
advantage of the fact that the `pool` attribute is accessed exactly
twice when running a task and putting a 1/0 payload in an overloaded
`pool` property. But that’s too fragile to make a unit test because no
one will be able to figure out why accessing `pool` elsewhere in
airflow makes this unit test fail. For the time being, I’m removing the
unit test.

tests/jobs.py

@@ -71,41 +71,6 @@ def test_backfill_examples(self):
                 ignore_first_depends_on_past=True)
             job.run()
 
-    def test_trap_executor_error(self):
-        """
-        Test that errors setting up tasks (before tasks run) are caught.
-
-        Executors run tasks with the `airflow run` command. If a task runs,
-        its state (success, failure, or other) is stored in the database and
-        `airflow run` exits without error. However, if an error is raised
-        before the task runs, then the task won't be able to update its status.
-        This can put the executor into an infinite loop of trying to run the
-        task.
-
-        To counteract that, the executor traps errors coming from
-        `airflow run` (essentially looking for returncode != 0).
-        This unit test creates such an error by trying
-        to run a subdag whose dag_id has changed and therefore can't be
-        found. If the trap is working properly, the error will be caught
-        and the Backfill will report failures. If the trap is not working,
-        the job will run infinitely (the unit test uses a timeout to protect
-        against that case).
-
-        Test for https://github.com/airbnb/airflow/pull/1220
-        """
-        dag = self.dagbag.get_dag('test_raise_executor_error')
-        dag.clear()
-        job = BackfillJob(
-            dag=dag,
-            start_date=DEFAULT_DATE,
-            end_date=DEFAULT_DATE)
-        # run with timeout because this creates an infinite loop if not
-        # caught
-        def run_with_timeout():
-            with timeout(seconds=30):
-                job.run()
-        self.assertRaises(AirflowException, run_with_timeout)
-
     def test_backfill_pooled_tasks(self):
         """
         Test that queued tasks are executed by BackfillJob