INTEGRATION_SceneBroadcaster is flaky

[mjcarroll]

Environment

• In Gazebo CI environments (Linux, Windows, and Homebrew)

Description

The test is frequently flaky in CI, for example: https://build.osrfoundation.org/job/ignition_gazebo-ci-pr_any-ubuntu_auto-amd64/9200/testReport/junit/(root)/ServerRepeat_SceneBroadcasterTest/AddRemoveEntitiesComponents_0/

Most of the flakes are centered around some variant of this:

/home/jenkins/workspace/ignition_gazebo-ci-pr_any-ubuntu_auto-amd64/ign-gazebo/test/integration/scene_broadcaster_system.cc:809
Expected equality of these values:
  periodicChangeIteration || oneTimeChangeIteration
    Which is: true
  hasModifiedComponent
    Which is: false

Example from Linux-actions-focal:

https://github.com/gazebosim/gz-sim/runs/7396548620?check_suite_focus=true

[mjcarroll]

I am working on a fix for this, I just wanted to have a reference to point to.