👩‍🌾 Improve velocity control test

[chapulina]

🦟 Bug fix

Summary

The PublishCmd test is slightly flaky, it failed 1 of the last 15 builds with:

127: /var/lib/jenkins/workspace/ignition_gazebo-ci-ign-gazebo3-bionic-amd64/ign-gazebo/test/integration/velocity_control_system.cc:128: Failure
127: Expected: (poses[i].Pos().X()) > (poses[i-1].Pos().X()), actual: 2.12913e-10 vs 2.13556e-10
127: /var/lib/jenkins/workspace/ignition_gazebo-ci-ign-gazebo3-bionic-amd64/ign-gazebo/test/integration/velocity_control_system.cc:129: Failure
127: Expected: (poses[i].Pos().Y()) > (poses[i-1].Pos().Y()), actual: 2 vs 2

I suspect this may happen when the message is received a bit too late and the first iteration misses the command. I added a little sleep for the lack of some better check. The test isn't very flaky, so this may be just enough.

I also cleaned up the test a bit. The sleep / maxSleep weren't being used, and there's no need to create a system to publish the message.

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

---

https://github.com/osrf/buildfarmer/issues/156

[codecov]

Codecov Report

Merging #642 (41e1c1f) into ign-gazebo3 (3465bbe) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-gazebo3     #642      +/-   ##
===============================================
- Coverage        77.72%   77.71%   -0.01%     
===============================================
  Files              210      210              
  Lines            11615    11615              
===============================================
- Hits              9028     9027       -1     
- Misses            2587     2588       +1     

Impacted Files	Coverage Δ
...e/ignition/gazebo/detail/EntityComponentManager.hh	95.00% <0.00%> (-0.72%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3465bbe...41e1c1f. Read the comment docs.