Fix LTTng issue with test_ros2trace and test_tracetools_launch so that they can run tracing tests in parallel

[christophebedard]

This is a follow-up to #94.

#95 changed the tracing tests to be able to run them in parallel with normal tests and tracing tests. However, there was still an issue when test_ros2trace and test_tracetools_launch were running tests in parallel in ci.ros2.org, see: #95 (comment). This issue does not seem to happen locally. If test_ros2trace and test_tracetools_launch happen to not run tests simultaneously, then the tests usually pass, but that is mostly based on luck.

To temporarily work around it, we made test_ros2trace depend on test_tracetools_launch so that they wouldn't run tests in parallel, see: #95 (comment).

Now, we need to fix the issue and remove the workaround.