tests.system: test_listing_topic_subscriptions[rest-rest] failed

Note: #1208 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky. ---- commit: bea7f781d4c2e44da40ebe1e44f14559458de375 buildURL: [Build Status](https://source.cloud.google.com/results/invocations/8ce8c763-90a3-4411-965c-b241b52040d6), [Sponge](http://sponge2/8ce8c763-90a3-4411-965c-b241b52040d6) status: failed