Fix C++ testing tutorial: comment out placeholder target_link_libraries > Cpp Testing #2833 (backport #6766)

[mergify]

Description

• The CMakeLists.txt snippet in the C++ testing tutorial includes target_link_libraries(${PROJECT_NAME}_tutorial_test name_of_local_library). name_of_local_library is a placeholder, so a literal copy-paste of the snippet fails to build (CMake can't find a target by that name).
• This PR comments the line out and adds one sentence below the code block explaining that it's optional

Fixes # (issue)
Fixes ros2/lyrical_tutorial_party#2833

Did you use Generative AI?

yes, Claude

Additional Information

cc @kscottz

---

This is an automatic backport of pull request #6766 done by Mergify.

[mergify]

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