Re-enable flaky PSM test

[sea-bass]

Description

Re-enables a flaky Planning Scene Monitor test, adding a wait between the thread being started in the SetUp() to avoid race conditions I could reproduce locally.

Terminal 1:

stress -c 16 --vm 20 --vm-bytes 256M --timeout 10000 

Terminal 2:

ros2 test /home/sebastian/workspace/moveit_ws/src/moveit2/moveit_ros/planning/planning_scene_monitor/test/launch/planning_scene_monitor.test.py test_binary_dir:=/home/sebastian/workspace/moveit_ws/build/moveit_ros_planning/planning_scene_monitor

See log of retries being tracked here: https://github.com/moveit/moveit2/actions/runs/11966295919/job/33361693036?pr=3124 -- humble CI failing for other reasons on main as of the last few days.

Mitigates (but does not close) #2821

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.58%. Comparing base (6d94dfb) to head (85f5c7f).
Report is 1 commits behind head on main.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3124      +/-   ##
==========================================
- Coverage   45.80%   45.58%   -0.21%     
==========================================
  Files         483      482       -1     
  Lines       40631    40432     -199     
==========================================
- Hits        18608    18428     -180     
+ Misses      22023    22004      -19     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests