[JTC] Fix the JTC length_error exceptions in the tests (backport #1360)

[mergify]

Fix the exception in JTC tests from time to time

     [INFO] [1730930292.973865185] [test_joint_trajectory_controller]: Using 'splines' interpolation method.
    [INFO] [1730930292.974352669] [test_joint_trajectory_controller]: Action status changes will be monitored at 20.00 Hz.
    terminate called after throwing an instance of 'std::length_error'
      what():  basic_string::_M_create

https://github.com/ros-controls/ros2_controllers/actions/runs/11712509515/job/32623343057?pr=1297

---

This is an automatic backport of pull request #1360 done by Mergify.

[mergify]

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	both modified:   joint_trajectory_controller/test/test_trajectory_controller_utils.hpp

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[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 65.05%. Comparing base (f91b264) to head (777cf15).
Report is 1 commits behind head on humble.

Files with missing lines	Patch %	Lines
...ntroller/test/test_trajectory_controller_utils.hpp	71.42%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           humble    #1361      +/-   ##
==========================================
+ Coverage   65.03%   65.05%   +0.01%     
==========================================
  Files         107      107              
  Lines       12374    12381       +7     
  Branches     7666     7669       +3     
==========================================
+ Hits         8047     8054       +7     
  Misses       1549     1549              
  Partials     2778     2778              

Flag	Coverage Δ
unittests	65.05% <71.42%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ntroller/test/test_trajectory_controller_utils.hpp	77.47% <71.42%> (-0.12%)	⬇️

... and 1 file with indirect coverage changes