Migrate to core24 and python 3.12

[upils]

Migrate imagecraft to core24 to avoid relying on 2 cores (since ubuntu-image is also now using core24) and so use python3.12 in the snap.

Python3.12 is explicitly used for now because otherwise python3.10 is picked up during the build. This will need to be removed at some point.

Also add a first very basic spread test to:

• make sure we test the snap is building and running fine
• prepare for more tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.60%. Comparing base (91048e5) to head (a0e7c48).
Report is 78 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   97.60%   97.60%   -0.01%     
==========================================
  Files          20       20              
  Lines         502      500       -2     
  Branches       81       81              
==========================================
- Hits          490      488       -2     
  Misses         10       10              
  Partials        2        2              

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

[upils]

Even though every CI check is green, the build of the snap is broken. This needs to be fixed before merging.

[upils]

This is blocked by canonical/craft-parts#786

[upils]

@sil2100 2 tests are still failing because we do not have the runners setup yet. Sergio from the snapd team should set them up today or this week.

[sil2100]

LGTM! Do we want to wait with merging it until Sergio is done? Also, left some inline comments, but all might be irrelevant (just questions).

[upils]

LGTM! Do we want to wait with merging it until Sergio is done? Also, left some inline comments, but all might be irrelevant (just questions).

Yes I would like to wait for the setup of the runners to make sure this is working fine. I copied the conf from ubuntu-image but I may have messed it up so I would like to be sure the whole thing will properly work before merging.

[upils]

@sil2100 I have added snapd-testing-tools as a subtree (like in ubuntu-image) so do not worry about the files added in tests/lib/external/. As previously mentioned, due to canonical/craft-parts#786, we cannot really merge this sadly because otherwise the tool will only work on 24.04.

Once we decide if we keep python3.11 or not, the code looks ready for a last review.

[sil2100]

I don't see anything controversial in these changes, LGTM per our discussions. Of course there's a lot, but it's all due to the snapd-testing-tools, as mentioned.