Fix JTC test speed scaling publisher (backport #2153)

[mergify]

I'm working on adding a test to the JTC but when running master locally it fails on my machine. I use CycloneDDS in my setup so that is likely why CI is passing (because FastDDS isn't enforcing this) but the output from the failure pointed to an easy fix.

When I run locally you can see the test publisher to /test_joint_trajectory_controller/speed_scaling_input has the wrong QoS settings...

3: [test_joint_trajectory_controller 1770662071.626853898]: Setting initial scaling factor to 0.123000
3: [test_joint_trajectory_controller 1770662071.626858865]: Using scaling state from the hardware from interface speed_scaling/speed_scaling_factor.
3: [test_joint_trajectory_controller 1770662071.627182014]: New publisher discovered on topic '/test_joint_trajectory_controller/speed_scaling_input', offering incompatible QoS. No messages will be sent to it. Last incompatible policy: DURABILITY_QOS_POLICY
3: /root/control_ws/src/ros2_controllers/joint_trajectory_controller/test/test_trajectory_controller.cpp:2835: Failure
3: Expected equality of these values:
3:   state->speed_scaling_factor
3:     Which is: 0.123
3:   0.765
3: 
3: [  FAILED  ] TrajectoryControllerTest.scaling_command_interface_sets_value (832 ms)
-------
3: [  FAILED  ] 2 tests, listed below:
3: [  FAILED  ] TrajectoryControllerTest.setting_scaling_factor_works_correctly
3: [  FAILED  ] TrajectoryControllerTest.scaling_command_interface_sets_value

because using rclcpp::SystemDefaultsQoS() lets the underlying RMW choose the settings, which almost always defaults to Volatile.

To fix this I set the publisher to Transient Local durability to ensure it doesn't miss the value changes published in the test.

---

This is an automatic backport of pull request #2153 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.15%. Comparing base (dae02b4) to head (8d39ae9).
⚠️ Report is 1 commits behind head on kilted.

Additional details and impacted files

@@           Coverage Diff           @@
##           kilted    #2162   +/-   ##
=======================================
  Coverage   85.15%   85.15%           
=======================================
  Files         151      151           
  Lines       14566    14569    +3     
  Branches     1260     1260           
=======================================
+ Hits        12403    12406    +3     
  Misses       1716     1716           
  Partials      447      447           

Flag	Coverage Δ
unittests	85.15% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...ory_controller/test/test_trajectory_controller.cpp	99.80% <100.00%> (+<0.01%)	⬆️

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