Feature request

Feature description

While creating tests for internal behaviours, we've reused some of the handy utility helpers in https://github.com/ros-planning/navigation2/tree/main/nav2_behavior_tree/test/utils. However, since those headers are behind a BUILD_TESTING flag, they're not installed in binary distributions, and so aren't possible to use in tests from another source package. It would be great to find a way to make those reusable in other packages.

Implementation considerations

IMO there's basically two approaches here:

• install the test/utility headers regardless of BUILD_TESTING. This leaks those headers outside of a test builds, but.. it's just a few headers
• Create a ~nav2_test_utils type of package, that contains these test utils, is a test_depends for nav2_behavior_tree and other packages that require these kinds of utilities.
• Suboptimally nothing, and we can duplicate the headers in our test packages, for better or worse.