nav2_util: don't export test_msgs dependency

[lopsided98]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses
Primary OS tested on	nixpkgs/nix-ros-overlay
Robotic platform tested on	N/A

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Description of contribution in a few bullet points

Since #2827, nav2_util no longer unconditionally depends on test_msgs, but was still exporting it as a dependency. This caused downstream packages to fail to build if test_msgs is not available.

cc @alsora

Description of documentation updates required from your changes

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Future work that may be required in bullet points

For Maintainers:

• Check that any new parameters added are updated in navigation.ros.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[mergify]

@lopsided98, please properly fill in PR template in the future. @SteveMacenski, use this instead.

• Check that any new parameters added are updated in navigation.ros.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists