👩‍🌾 Make GitHub actions tests that are flaky due to display more verbose

[chapulina]

🦟 Bug exploration

Summary

Our tests on ign-rendering and ign-gui occasionally fail on GitHub actions because they can't open the display, see more info on gazebosim/gz-gui#58.

This is an attempt of making the display tests more robust (with the new arguments passed to Xvfb) and failures more verbose (with the updated error messages on Ogre2RenderEngine).

Unfortunately, the failure is so rare that I haven't been able to get a failure message with this branch yet. It should come with time, unless the changes to the Xvfb invocation really fix it.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

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https://github.com/osrf/buildfarmer/issues/161

[codecov]

Codecov Report

Merging #255 (42d6758) into ign-rendering3 (64016f6) will decrease coverage by 0.00%.
The diff coverage is 33.33%.

@@                Coverage Diff                 @@
##           ign-rendering3     #255      +/-   ##
==================================================
- Coverage           50.59%   50.59%   -0.01%     
==================================================
  Files                 129      129              
  Lines               11882    11887       +5     
==================================================
+ Hits                 6012     6014       +2     
- Misses               5870     5873       +3     

Impacted Files	Coverage Δ
ogre2/src/Ogre2RenderEngine.cc	78.80% <33.33%> (-0.59%)	⬇️

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