tests/core: add test for the prepare-image switch and env variable

[Meulengracht]

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[github-actions]

Fri Feb 28 14:58:24 UTC 2025
The following results are from: https://github.com/canonical/snapd/actions/runs/13589799350

Failures:

Executing:

• google-core:ubuntu-core-20-64:tests/core/prepare-image-mismatch-snapd
• google-core:ubuntu-core-24-64:tests/core/prepare-image-mismatch-snapd

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.11%. Comparing base (a272aac) to head (cfb2157).
Report is 77 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15147      +/-   ##
==========================================
+ Coverage   78.07%   78.11%   +0.04%     
==========================================
  Files        1182     1178       -4     
  Lines      157743   158150     +407     
==========================================
+ Hits       123154   123540     +386     
+ Misses      26943    26934       -9     
- Partials     7646     7676      +30     

Flag	Coverage Δ
unittests	78.11% <ø> (+0.04%)	⬆️

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[ZeyadYasser]

Thank you!