test: avoid flake if connections were already open at start (#1096)
test: avoid flake if connections were already open at start (#1096)

Addresses flakes such as https://source.cloud.google.com/results/invocations/bb7ce2a8-d410-45bd-af12-7c04bb8cbb25/targets/cloud-devrel%2Fclient-libraries%2Fpython%2Fgoogleapis%2Fpython-bigquery%2Fpresubmit%2Fsystem-3.10/log
googleapis/python-bigquery,googleapis/python-bigquery