Refactor CPU measurement utilities and tests

[mm318]

This is a refactoring of functions and tests related to system CPU usage measurement, moving some of them to a common area to be shared by process CPU usage measurement later.

[codecov]

Codecov Report

Merging #45 into master will increase coverage by 0.03%.
The diff coverage is 24%.

@@            Coverage Diff            @@
##           master     #45      +/-   ##
=========================================
+ Coverage   41.66%   41.7%   +0.03%     
=========================================
  Files          27      28       +1     
  Lines         876     880       +4     
  Branches      504     506       +2     
=========================================
+ Hits          365     367       +2     
  Misses         52      52              
- Partials      459     461       +2

Flag	Coverage Δ
#unittests	41.7% <24%> (+0.03%)	⬆️

Impacted Files	Coverage Δ
...m_metrics_collector/linux_cpu_measurement_node.cpp	34.61% <ø> (-2.12%)	⬇️
...m_metrics_collector/linux_cpu_measurement_node.hpp	100% <ø> (ø)	⬆️
...m_metrics_collector/test_linux_cpu_measurement.cpp	28.8% <0%> (+2.71%)	⬆️
...r/test/system_metrics_collector/test_utilities.cpp	14.89% <17.64%> (+0.6%)	⬆️
...r/test/system_metrics_collector/test_utilities.hpp	25% <25%> (ø)
...llector/src/system_metrics_collector/utilities.cpp	33.8% <42.3%> (+4.91%)	⬆️

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[thomas-moulard]

LGTM