Fix joint controller GUI test

[chapulina]

🦟 Bug fix

Summary

The JoingPositionController_TEST was broken when #534 was forward-ported in #689.

I think this fix is also reasonable for ign-gazebo3, but for some timing issue, the test doesn't seem to be failing there. So I'm fixing it here and we can backport if needed.

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

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[codecov]

Codecov Report

Merging #697 (99698a6) into ign-gazebo4 (302f5ed) will decrease coverage by 11.85%.
The diff coverage is 68.55%.

@@               Coverage Diff                @@
##           ign-gazebo4     #697       +/-   ##
================================================
- Coverage        77.37%   65.51%   -11.86%     
================================================
  Files              217      237       +20     
  Lines            12217    17197     +4980     
================================================
+ Hits              9453    11267     +1814     
- Misses            2764     5930     +3166     

Impacted Files	Coverage Δ
include/ignition/gazebo/EntityComponentManager.hh	100.00% <ø> (ø)
include/ignition/gazebo/rendering/SceneManager.hh	100.00% <ø> (ø)
src/Conversions.cc	82.03% <ø> (+0.13%)	⬆️
.../plugins/component_inspector/ComponentInspector.cc	7.29% <0.00%> (-1.32%)	⬇️
.../plugins/component_inspector/ComponentInspector.hh	28.57% <ø> (ø)
src/network/PeerTracker.hh	100.00% <ø> (ø)
src/systems/apply_joint_force/ApplyJointForce.hh	100.00% <ø> (ø)
src/systems/breadcrumbs/Breadcrumbs.hh	100.00% <ø> (ø)
src/systems/buoyancy/Buoyancy.hh	100.00% <ø> (ø)
src/systems/contact/Contact.hh	100.00% <ø> (ø)
... and 82 more

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