Fix UNIT_Server_TEST failure caused by change in behavior of gz::common::SignalHandler

[azeey]

🦟 Bug fix

Summary

gazebosim/gz-common#618 changed the way gz::common::SignalHandler works in that the callback is invoked from a separate thread. This causes a slight difference in the timing of when callbacks get called. Adding a small delay after raising a signal fixes the problem.

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)
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[iche033]

I was able to reproduce the test failures after updating gz-common, and confirms that this fixes the test for me.

[iche033]

CI confirms that UNIT_Server_TEST test failures are fixed. The INTEGRATION_mesh_inertia_calculation test failure on windows should be unrelated. Merging.