Fix performance level test flakiness

[nkoenig]

Signed-off-by: Nate Koenig [email protected]

🎉 New feature

Closes #826

Summary

Fixes the PERFORMANCE_level_manager by running the server once before measuring.

Test it

Run the test with and without the change. You should see a big difference in the time Using levels = X ms.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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

[codecov]

Codecov Report

Merging #1129 (38784b7) into ign-gazebo3 (0640db9) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-gazebo3    #1129   +/-   ##
============================================
  Coverage        77.70%   77.70%           
============================================
  Files              222      222           
  Lines            12716    12716           
============================================
  Hits              9881     9881           
  Misses            2835     2835           

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