Fix flaky test_mount_unmount_zfs_file_system()

[pem70]

What It Does
Fixes #383

How to Test
Run test_mount_unmount_zfs_file_system() and make it fail before unmounting. The created file system should still be unmounted and deleted.

Review Checklist
I certify that I have:

• tested my changes
• added/updated automated tests
• updated the changelog
• followed the contribution guidelines

Additional Comments

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.86%. Comparing base (7d093e6) to head (9979940).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #384   +/-   ##
=======================================
  Coverage   81.86%   81.86%           
=======================================
  Files          48       48           
  Lines        2768     2768           
=======================================
  Hits         2266     2266           
  Misses        502      502           

Flag	Coverage Δ
unittests	81.86% <ø> (ø)

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