[JTC] Fix state offset tests (backport #797)

[mergify]

This is an automatic backport of pull request #797 done by Mergify.
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Changes to be committed:
	modified:   joint_trajectory_controller/include/joint_trajectory_controller/joint_trajectory_controller.hpp
	modified:   joint_trajectory_controller/src/joint_trajectory_controller.cpp
	modified:   joint_trajectory_controller/test/test_trajectory_controller.cpp

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	both modified:   joint_trajectory_controller/test/test_trajectory_controller_utils.hpp

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[christophfroehlich]

Needs a backport of #834 first

[christophfroehlich]

Needs a backport of #834 first

i.e., #843