tests: retry installing lxd snap

[mr-cal]

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• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

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Use the retry function from snapd-testing-tools when installing the lxd snap.

This is to workaround an intermittent and hard-to-reproduce snapd bug.

⚠️ If you are having trouble running spread tests: Try fetching the git submodules (git submodule update --init --recursive) to collect the snapd-testing-tools submodule.

[codecov-commenter]

Codecov Report

Merging #3957 (c9431b4) into main (3410375) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3957   +/-   ##
=======================================
  Coverage   91.06%   91.06%           
=======================================
  Files          56       56           
  Lines        4098     4098           
=======================================
  Hits         3732     3732           
  Misses        366      366           

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