ci: shard alpine-test into parallel jobs to reduce CI time

[adrianreber]

The alpine-test CI job runs all ~483 zdtm tests sequentially three times (normal, mntns-compat-mode, criu-config), followed by many non-shardable tests. This dominates overall CI wait time. With only 2 jobs running in parallel (GCC and CLANG) the alpine tests take around 30 minutes.

Use the existing --test-shard-index and --test-shard-count flags already built into test/zdtm.py to split the zdtm test suite across four parallel runners (shards 0-3). A fifth shard runs all non-shardable tests (lazy pages, fault injection, test/others/*, rootless, compel, plugins, etc.) independently and in parallel with the zdtm shards. This increases parallelism from 2 to 10 jobs and reduces the alpine test wall-clock time from ~30 to ~10 minutes.

Changes:

• run-ci-tests.sh: Build SHARD_OPTS from ZDTM_SHARD_INDEX/COUNT env vars and pass them to zdtm.py. Extract all non-shardable tests into a run_non_shardable_tests() function. Dispatch based on shard index: 0-3 run zdtm slices, 4 runs non-shardable tests, unset runs everything sequentially (preserving existing behavior).
• Makefile: Pass ZDTM_SHARD_INDEX and ZDTM_SHARD_COUNT into the container when set.
• ci.yml: Add shard: [0, 1, 2, 3, 4] to the alpine-test matrix, producing 10 jobs (2 compilers x 5 shards). Job labels now show descriptive shard names (e.g. "zdtm 1/4", "non-zdtm") instead of raw indices.

When sharding is not configured the script behaves identically to before, so other CI jobs (aarch64, compat, gcov, etc.) are unaffected.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.52%. Comparing base (5546c06) to head (e1ddb0e).
⚠️ Report is 4 commits behind head on criu-dev.

Additional details and impacted files

@@             Coverage Diff              @@
##           criu-dev    #2966      +/-   ##
============================================
+ Coverage     57.21%   57.52%   +0.31%     
============================================
  Files           154      158       +4     
  Lines         40400    40448      +48     
  Branches       8856     8864       +8     
============================================
+ Hits          23113    23268     +155     
- Misses        17023    17036      +13     
+ Partials        264      144     -120     

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[avagin]

LGTM, thanks.

An alternative solution would be to splice run-ci-tests.sh. We don't need to run all these tests in one job. It can be separate jobs to run zdtm tests, pre-dump tests, fault tests, etc.

[github-actions]

A friendly reminder that this PR had no activity for 30 days.