test: sync fs before checking for output file

[pb8o]

It can happen that we check for the file before socat has written it in the filesystem.

Sync before checking.

Changes

...

Reason

...

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 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 83.13%. Comparing base (d28ef60) to head (c62923e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5010   +/-   ##
=======================================
  Coverage   83.13%   83.13%           
=======================================
  Files         245      245           
  Lines       26697    26697           
=======================================
  Hits        22194    22194           
  Misses       4503     4503           

Flag	Coverage Δ
5.10-c5n.metal	83.59% <ø> (-0.01%)	⬇️
5.10-m5n.metal	83.58% <ø> (ø)
5.10-m6a.metal	82.79% <ø> (-0.01%)	⬇️
5.10-m6g.metal	79.56% <ø> (ø)
5.10-m6i.metal	83.57% <ø> (-0.01%)	⬇️
5.10-m7g.metal	79.56% <ø> (ø)
6.1-c5n.metal	83.59% <ø> (ø)
6.1-m5n.metal	83.58% <ø> (ø)
6.1-m6a.metal	82.79% <ø> (ø)
6.1-m6g.metal	79.56% <ø> (ø)
6.1-m6i.metal	83.57% <ø> (-0.01%)	⬇️
6.1-m7g.metal	79.56% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.