Fix flakiness and formatting issues in google-cloud-pubsub schemas acceptance tests #34025

Environment details

• OS: Linux (Kokoro run) (see: https://btx.cloud.google.com/invocations/6057621a-4719-4302-a701-bf973c011ec7/targets/cloud-devrel%2Fclient-libraries%2Fgoogle-cloud-ruby%2Fpresubmit%2Fsamples/log)
• Ruby version: 3.2-4.0
• Gem name and version: google-cloud-pubsub

Steps to reproduce

1. I submitted chore: Add pubsub rollback schema sample #34019
2. I noticed test failures in schemas/acceptance_test.rb; the PR changes did not overlap with those tests, so these seem to be flaky failures that affect throughput. This issue is to track the fix to address this CI flakiness.

Code example

    expect_with_retry "pubsub_subscribe_avro_records_with_revisions" do
        assert_output /Received a binary-encoded message:.*Alaska.*Received a binary-encoded message:.*California/m do
          subscribe_avro_records_with_revisions subscription_id: @subscription.name
        end
      end

Full backtrace

Finished in 1505.708359s, 0.0345 runs/s, 0.1561 assertions/s.

  1) Failure:
schemas::AVRO#test_0006_supports pubsub_subscribe_avro_records_with_revisions [acceptance/schemas_test.rb:265]:
In stdout.
Expected /Received a binary-encoded message:.*Alaska.*Received a binary-encoded message:.*California/m to match "".

52 runs, 235 assertions, 1 failures, 0 errors, 8 skips