Fix flaky triggered camera integration test

[iche033]

🦟 Bug fix

Fixes #1894

Summary

The triggered camera integration test is flaky in CI builds. Occasionally requests fail to trigger camera callbacks.

The console output suggests that the tests are being run before the rendering thread is initialized. So this PR attempts to fix the test by adding logic to wait for render events to be fired first (this means rendering thread is ready) before proceeding to check for triggered camera callbacks.

Console output from failed CI run - last two lines indicate rendering thread initialized too late:

243: [Dbg] [Sensors.cc:240] Initializing render context
243: [Msg] Loading plugin [ignition-rendering-ogre2]
243: /home/jenkins/workspace/ignition_gazebo-ci-ign-gazebo6-focal-amd64/ign-gazebo/test/integration/triggered_camera.cc:105: Failure
243: Expected: (sleep) < (maxSleep), actual: 30 vs 30
243: /home/jenkins/workspace/ignition_gazebo-ci-ign-gazebo6-focal-amd64/ign-gazebo/test/integration/triggered_camera.cc:106: Failure
243: Expected: (imageBuffer) != (nullptr), actual: NULL vs (nullptr)
243: [Dbg] [Sensors.cc:406] SensorsPrivate::Stop
243: [Dbg] [RenderUtil.cc:2542] Create scene [scene]
243: [Dbg] [Sensors.cc:254] Rendering Thread initialized

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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