joint_state_broadcaster: Add proper subscription to TestCustomInterfaceMappingUpdate

[christophfroehlich]

As mentioned with #821, one tests of joint_state_broadcaster is flaky in the coverage build job. This is because the creation of the joint_state_msg is not ensured due to a failing trylock() when calling update() only once

ros2_controllers/joint_state_broadcaster/src/joint_state_broadcaster.cpp

Line 353 in 0d3fc52

if (realtime_joint_state_publisher_ && realtime_joint_state_publisher_->trylock())

I reused the wait set from other tests, which retries the update several times.

Unfortunately, I was not able to reproduce a failing test like in the CI, but this should fix #821 (other tests using the wait_set are not flaky).

[codecov]

Codecov Report

Merging #859 (6c48ae2) into master (0d3fc52) will increase coverage by 0.44%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #859      +/-   ##
==========================================
+ Coverage   44.88%   45.32%   +0.44%     
==========================================
  Files          40       40              
  Lines        3636     3636              
  Branches     1716     1716              
==========================================
+ Hits         1632     1648      +16     
+ Misses        832      817      -15     
+ Partials     1172     1171       -1     

Flag	Coverage Δ
unittests	45.32% <ø> (+0.44%)	⬆️

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see 3 files with indirect coverage changes

[bmagyar]

Thank you!