Re-enable planning_scene_monitor.test.py test after diagnosing its flaky bring-up

[EzraBrooks]

This is failing across a number of branches. It may be a latency issue, esp. if that latency is induced specifically by the slowness of GitHub Actions runners, since we can't seem to reproduce this locally.

[github-actions]

This issue is being labeled as stale because it has been open 45 days with no activity. It will be automatically closed after another 45 days without follow-ups.

[sea-bass]

It appears this test is still enabled on the humble branch and very much flaky.

Rather than disabling it there, I wonder if it's a good time to give this a look.

Whenever I see flaky tests like this, I tend to pull out the old stress command to recreate the CI runners. It's paid off a few times.

[sea-bass]

Note that I've re-enabled the test after adding a 1 second sleep after kicking off the executor thread in the fixture setup, which somehow resolves a race condition and seems to make the test pass consistently (I ran them many times in CI).

#3124

I would not consider this issue closed until the real root cause is resolved, though.