Restore exported BT test utils header files after cmake revamp (#4652)

[aosmw]

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#4652
Primary OS tested on	(Ubuntu)
Robotic platform tested on	n/a
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

Restored installation of nav2_behaviour_tree test utils header files after cmake revamp.

These are needed to facilitate testing of custom actions/conditions etc.

Description of documentation updates required from your changes

n/a

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Future work that may be required in bullet points

n/a

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 4 files with indirect coverage changes