Address flaky UNIT_Gui_TEST

[nkoenig]

🦟 Bug fix

Summary

This improves the stability of the UNIT_Gui_TEST. I tested by running the following command:

for i in {1..1000}; do gdb -ex=r -ex=q ./bin/UNIT_Gui_TEST; done

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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1776 (4a766f0) into gz-sim7 (02b3f59) will decrease coverage by 0.00%.
The diff coverage is 67.77%.

❗ Current head 4a766f0 differs from pull request most recent head b2a154a. Consider uploading reports for the commit b2a154a to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #1776      +/-   ##
===========================================
- Coverage    64.22%   64.22%   -0.01%     
===========================================
  Files          336      336              
  Lines        26603    26853     +250     
===========================================
+ Hits         17087    17245     +158     
- Misses        9516     9608      +92     

Impacted Files	Coverage Δ
include/gz/sim/SystemLoader.hh	100.00% <ø> (ø)
include/gz/sim/detail/EntityComponentManager.hh	93.86% <ø> (ø)
include/gz/sim/rendering/RenderUtil.hh	100.00% <ø> (ø)
.../plugins/component_inspector/ComponentInspector.hh	28.57% <ø> (ø)
src/gui/plugins/scene_manager/GzSceneManager.cc	17.58% <0.00%> (-0.60%)	⬇️
...rc/systems/ackermann_steering/AckermannSteering.hh	100.00% <ø> (ø)
.../systems/triggered_publisher/TriggeredPublisher.hh	100.00% <ø> (ø)
.../plugins/component_inspector/ComponentInspector.cc	5.46% <2.77%> (-0.33%)	⬇️
...s/multicopter_motor_model/MulticopterMotorModel.cc	75.91% <33.33%> (-0.22%)	⬇️
.../systems/triggered_publisher/TriggeredPublisher.cc	83.33% <84.28%> (-0.39%)	⬇️
... and 12 more

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[azeey]

Other than the potential for flakiness due to spurious wakeups, this looks good. I tested with ctest -R ^UNIT_Gui_TEST --repeat-until-fail 100 and didn't see any failures of the Quickstart test. I did see the PathManager fail though :(