Fix race condition in websocket test (backport #1185)

[mergify]

Public API Changes
None

Description
This fixes an issue with the websocket testing instrumentation which had a race condition inherent in it that was causing my test to sometimes fail incorrectly (false negative) which led me to believe that other solutions were helping with other issues (they were not).

Changes:

1. Fix websocket testing instrumentation race condition when many clients requested at once. You can verify this fix by trying this before and after the fix:

colcon test --packages-select rosbridge_server --ctest-args -R "test_startup_race" --repeat until-fail:500 --event-handlers console_direct+ 2>&1 | tee /tmp/startup_race_retry.txt
# in another terminal:
stress --cpu 15

Eventually this should fail, especially with stress running at the same time (load your CPU as much as possible). The fix was to use the reactorThread for sending messages and to add retries to getting the ros port parameter from the ros node.
3. Removes the startup_race.test.py test which, it turns out, was mostly exercising the websocket test instrumentation and not any particularly useful code paths.
4. Reverts the previous "fix" I made to the server instantiation order. Upon further examination, I realized that the order does not matter since the websocket requests are not processed in rosbridge_websocket.py until we call await stop_event.wait(), which is already after the ros2 executor and node are all started. I can remove this change if we want to keep the order of instantiation I had changed to earlier (either way should work fine).

---

This is an automatic backport of pull request #1185 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	modified:   rosbridge_server/scripts/rosbridge_websocket.py
	modified:   rosbridge_server/test/websocket/common.py

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   rosbridge_server/CMakeLists.txt
	deleted by them: rosbridge_server/test/websocket/startup_race.test.py

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