Add fast integration tests for cluv submit that can run in CI

[lebrice]

The existing real-cluster test_submit is skipped in CI (it waits for the job to actually run), so the real sync → sbatch path of cluv submit has no coverage on Codecov.

This adds two integration tests that exercise that path in seconds:

• test_submit_held_job: submits scripts/job.sh with --hold. A held job stays PENDING, so submit returns as soon as it sees it's the only pending job (no waiting for resources). Checks that a Job is returned and sacct reports PENDING, then scancels it.
• test_submit_rejected_by_sbatch: submits with --partition=cluv-does-not-exist and checks that submit returns None when sbatch rejects the job.

Both follow test_submit's conventions (slow marker, xfail on clusters outside SUBMIT_SUPPORTED_CLUSTERS). In the self-hosted CI they'll run on mila; tamia xfails.

Tested locally: 4 passed (mila + rorqual) in ~2 min, no held jobs left in the queue.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.42%. Comparing base (5c25d22) to head (9c9f8f0).

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #224       +/-   ##
===========================================
+ Coverage   72.10%   87.42%   +15.31%     
===========================================
  Files          22       22               
  Lines        2370     2370               
===========================================
+ Hits         1709     2072      +363     
+ Misses        661      298      -363     

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