tests/system/test_client.py::TestBigQuery::test_transaction_info flakey

[tswast]

As mentioned in #958, it's hard to debug these further without a job ID, but I think it's worth looking into once we have enough information to share with the backend folks.

Additional failures from #961

https://source.cloud.google.com/results/invocations/00e1aa80-e4a9-4e30-a9cf-5a487bb374eb/targets/cloud-devrel%2Fclient-libraries%2Fpython%2Fgoogleapis%2Fpython-bigquery%2Fpresubmit%2Fprerelease-deps-3.8/log

https://source.cloud.google.com/results/invocations/435aecd6-5a1f-45ef-b975-3b2179be16a7/targets/cloud-devrel%2Fclient-libraries%2Fpython%2Fgoogleapis%2Fpython-bigquery%2Fpresubmit%2Fsystem-3.8/log

[tswast]

Seems the problem has been resolved. I haven't seen this flake in the past week.

[tswast]

Problem is still there: #975