Fix flaky backfill test (apache#28087)

The test falls sometimes when initial setup of the tests causes
recoverable failure - making "first" test contain different error
followed by an expected one.

The fix is to look for the message in all error logs registered

Co-authored-by: Andrey Anshin <Andrey.Anshin@taragol.is>

Co-authored-by: Andrey Anshin <Andrey.Anshin@taragol.is>

tests/jobs/test_backfill_job.py

@@ -636,9 +636,10 @@ def test_backfill_skip_active_scheduled_dagrun(self, dag_maker, caplog):
             start_date=DEFAULT_DATE,
             end_date=DEFAULT_DATE + datetime.timedelta(days=2),
         )
-        job.run()
-        error_log_records = [record for record in caplog.records if record.levelname == "ERROR"]
-        assert "Backfill cannot be created for DagRun" in error_log_records[0].msg
+        with caplog.at_level(logging.ERROR, logger="airflow.jobs.backfill_job.BackfillJob"):
+            caplog.clear()
+            job.run()
+            assert "Backfill cannot be created for DagRun" in caplog.messages[0]
 
         ti = TI(
             task=dag.get_task("test_backfill_skip_active_scheduled_dagrun-1"), execution_date=DEFAULT_DATE