tests: fix lxd-mount-units in ubuntu jammy

[sergiocazzolato]

The lxd image used in jammy is seeded with core22 snap.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.84%. Comparing base (17deb50) to head (d4a3d20).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #17404      +/-   ##
==========================================
- Coverage   78.93%   78.84%   -0.09%     
==========================================
  Files        1390     1404      +14     
  Lines      196047   196698     +651     
  Branches     2462     2462              
==========================================
+ Hits       154740   155084     +344     
- Misses      32024    32285     +261     
- Partials     9283     9329      +46     

Flag	Coverage Δ
unittests	78.84% <ø> (-0.09%)	⬇️

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[github-actions]

Fri Jul 24 06:33:14 UTC 2026
The following results are from: https://github.com/canonical/snapd/actions/runs/30061469998

Failures:

Skipped tests from snapd-testing-skip

If you wish to have any of the below tests run in your PR, in your PR description, add 'unskip:' followed by a copy-and-pasted list of the below tests you wish to run (unskip plus test list must be valid yaml)