Switch nav2_system_tests to modern CMake idioms.

[clalancette]

---

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	Follow-up to #4357
Primary OS tested on	Ubuntu 24.04
Robotic platform tested on	N/A
Does this PR contain AI generated software?	No

---

Description of contribution in a few bullet points

Update nav2_system_tests to use modern CMake idioms:

1. Make everything in this package a test_depend, since this is all tests.
2. Along with the previous point, move everything in this package under the BUILD_TESTING conditional in CMake.
3. Switch from ament_target_dependencies to target_link_libraries everywhere.
4. Remove all references to Boost.

Description of documentation updates required from your changes

None needed.

---

Future work that may be required in bullet points

This is part of a larger series updating Navigation2 to use modern CMake idioms. After this PR, there are 2 PRs left to open.

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

[clalancette]

I should also mention; with this in, my compile time savings locally is about 1 minute. This is substantially less than I originally saw in #4357; I suspect that there is some other bug on the iron branch that makes that one really slow. Regardless, I think we accomplished a lot with this series in terms of cleanup, so I'm happy we did it.

I'm also going to close #4357 at this point, as with this PR we accomplish everything that one set out to, and more.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

[SteveMacenski]

I also noticed that the WPF wasn't updated https://github.com/ros-navigation/navigation2/blob/main/nav2_waypoint_follower/CMakeLists.txt - I don't think that's probably the difference maker in compile times, but would you mind doing the last one so they're all homogeneous?

[clalancette]

I also noticed that the WPF wasn't updated https://github.com/ros-navigation/navigation2/blob/main/nav2_waypoint_follower/CMakeLists.txt - I don't think that's probably the difference maker in compile times, but would you mind doing the last one so they're all homogeneous?

Yeah, that's one of the last 2 PRs I have (the other is just some minor cleanup here and there). I left it to last because it also requires an update to robot_localization, which I haven't opened the PR for yet either. I'll do that soon.