Fix use of mutable default arg in tracetools_test

[christophebedard]

This issue only affects tracetools_test-based tests if multiple tests are run in the same pytest invocation (i.e., ament_add_pytest_test()). The issue is that the default list value for additional_actions is mutable. We kept appending to the same list, so we re-used Trace action objects for multiple LaunchService.run() calls, and ended up performing substitutions twice, which was raising an error.

I fixed this by changing the default value to None and then changing None to [] if needed. I also changed additional_actions to be appended instead of prepended to the list of launch actions, since the Trace action must be first; this itself would've fixed the issue too.

I also simplified the typing a bit. This additional_actions argument isn't actually used anywhere here, so it could technically be removed, but we can just keep it.

Note that test_tracetools currently has individual ament_add_pytest_test() calls for its tests, but I've bumped into this issue internally.

[clalancette]

Looks reasonable to me with green CI.

[christophebedard]

• Linux
• Linux-aarch64
• Windows

[christophebedard]

CI looks good. Thank you for the review.