cpu monitor test fix (backport #618 to ros2-humble)

[mergify]

Signed-off-by: Christian Henkel christian.henkel2@de.bosch.com

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This is an automatic backport of pull request #618 done by Mergify.

[mergify]

Cherry-pick of fa062d0 has failed:

On branch mergify/bp/ros2-humble/pr-618
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Unmerged paths:
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	both modified:   diagnostic_common_diagnostics/test/systemtest/test_cpu_monitor.py

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