fix(test/cpu_monitor): use psutil to get more accurate CPU measurements

[Manciukic]

Changes

We were using top to gather CPU usage of the threads of Firecracker. Replace it with a more sane approach which uses psutil.

This also fixes a bug due to the CPU monitor lasting longer than the test itself.

Reason

Make CPU monitor more accurate and fix bug of monitor measuring beyond test duration.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.35%. Comparing base (1253957) to head (236b61f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5363      +/-   ##
==========================================
+ Coverage   82.30%   82.35%   +0.04%     
==========================================
  Files         265      265              
  Lines       30640    30640              
==========================================
+ Hits        25219    25233      +14     
+ Misses       5421     5407      -14     

Flag	Coverage Δ
5.10-c5n.metal	82.32% <ø> (ø)
5.10-m5n.metal	82.31% <ø> (-0.01%)	⬇️
5.10-m6a.metal	81.60% <ø> (-0.01%)	⬇️
5.10-m6g.metal	78.92% <ø> (ø)
5.10-m6i.metal	82.32% <ø> (ø)
5.10-m7a.metal-48xl	81.59% <ø> (?)
5.10-m7g.metal	78.92% <ø> (ø)
5.10-m7i.metal-24xl	82.28% <ø> (?)
5.10-m7i.metal-48xl	82.29% <ø> (?)
5.10-m8g.metal-24xl	78.91% <ø> (?)
5.10-m8g.metal-48xl	78.92% <ø> (?)
6.1-c5n.metal	82.36% <ø> (+<0.01%)	⬆️
6.1-m5n.metal	82.36% <ø> (+<0.01%)	⬆️
6.1-m6a.metal	81.65% <ø> (ø)
6.1-m6g.metal	78.92% <ø> (ø)
6.1-m6i.metal	82.36% <ø> (ø)
6.1-m7a.metal-48xl	81.63% <ø> (?)
6.1-m7g.metal	78.92% <ø> (ø)
6.1-m7i.metal-24xl	82.37% <ø> (?)
6.1-m7i.metal-48xl	82.38% <ø> (?)
6.1-m8g.metal-24xl	78.91% <ø> (?)
6.1-m8g.metal-48xl	78.91% <ø> (?)

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