[tests] Network and cpu utilization improvements

[ShadowCurse]

Changes

Minor clean up of network and cpu utilization testing code. Now cpu utilization is tracked for all threads.

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

• [ ] If a specific issue led to this PR, this PR closes the issue.
• [ ] The description of changes is clear and encompassing.
• [ ] Any required documentation changes (code and docs) are included in this PR.
• [ ] API changes follow the Runbook for Firecracker API changes.
• [ ] User-facing changes are mentioned in CHANGELOG.md.
• [ ] All added/changed functionality is tested.
• [ ] New TODOs link to an issue.
• [ ] Commits meet contribution quality standards.

---

• [ ] This functionality cannot be added in rust-vmm.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (21f0dea) 81.63% compared to head (a75a2ac) 81.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4456   +/-   ##
=======================================
  Coverage   81.63%   81.63%           
=======================================
  Files         243      243           
  Lines       29789    29789           
=======================================
  Hits        24319    24319           
  Misses       5470     5470           

Flag	Coverage Δ
4.14-c5n.metal	78.95% <ø> (?)
4.14-c7g.metal	?
4.14-m5d.metal	?
4.14-m5n.metal	78.94% <ø> (?)
4.14-m6a.metal	78.08% <ø> (ø)
4.14-m6g.metal	77.03% <ø> (ø)
4.14-m6i.metal	78.94% <ø> (ø)
4.14-m7g.metal	77.03% <ø> (?)
5.10-c5n.metal	81.61% <ø> (?)
5.10-c7g.metal	?
5.10-m5d.metal	?
5.10-m5n.metal	81.60% <ø> (?)
5.10-m6a.metal	80.83% <ø> (ø)
5.10-m6g.metal	79.92% <ø> (ø)
5.10-m6i.metal	81.59% <ø> (ø)
5.10-m7g.metal	79.92% <ø> (?)
6.1-c5n.metal	81.61% <ø> (?)
6.1-m5n.metal	81.60% <ø> (?)
6.1-m6a.metal	80.83% <ø> (ø)
6.1-m6g.metal	79.92% <ø> (ø)
6.1-m6i.metal	81.59% <ø> (ø)
6.1-m7g.metal	79.92% <ø> (?)

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