Fix flaky bounding box camera test

[iche033]

🦟 Bug fix

Fixes #328

Summary

Fixes flaky bounding box camera test due to timing / race condition. This PR fixes the test by making sure:

1. we allow time for trigger msg to arrive to bounding box camera
2. make sure msgs are received on the box topic (instead of image topic) which is what the tests is checking
3. add mutex to protect the isTriggered variable.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #329 (7421a1d) into gz-sensors7 (0bab350) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 7421a1d differs from pull request most recent head 651e1fb. Consider uploading reports for the commit 651e1fb to get more accurate results

@@             Coverage Diff              @@
##           gz-sensors7     #329   +/-   ##
============================================
  Coverage        69.97%   69.98%           
============================================
  Files               36       36           
  Lines             3920     3921    +1     
============================================
+ Hits              2743     2744    +1     
  Misses            1177     1177           

Impacted Files	Coverage Δ
src/BoundingBoxCameraSensor.cc	61.02% <100.00%> (+0.11%)	⬆️

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