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

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). Validate that ZDTM_SHARD_INDEX is set when
  ZDTM_SHARD_COUNT is set.
- Makefile: Pass ZDTM_SHARD_INDEX and ZDTM_SHARD_COUNT into the
  container when set. Split long container run command across
  multiple lines for readability.
- 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.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Adrian Reber <areber@redhat.com>

.github/workflows/ci.yml

@@ -9,16 +9,31 @@ concurrency:
 
 jobs:
   alpine-test:
-    name: Alpine Test
+    name: Alpine Test (${{ matrix.target }}, ${{ matrix.shard_name }})
     strategy:
       matrix:
         os: [ubuntu-22.04]
         target: [GCC=1, CLANG=1]
+        shard: [0, 1, 2, 3, 4]
+        include:
+          - shard: 0
+            shard_name: zdtm 1/4
+          - shard: 1
+            shard_name: zdtm 2/4
+          - shard: 2
+            shard_name: zdtm 3/4
+          - shard: 3
+            shard_name: zdtm 4/4
+          - shard: 4
+            shard_name: non-zdtm
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4
-    - name: Run Alpine ${{ matrix.target }} Test
-      run: sudo -E make -C scripts/ci alpine ${{ matrix.target }}
+    - name: Run Alpine ${{ matrix.target }} ${{ matrix.shard_name }} Test
+      run: >
+        sudo -E make -C scripts/ci alpine ${{ matrix.target }}
+        ZDTM_SHARD_INDEX=${{ matrix.shard }}
+        ZDTM_SHARD_COUNT=4
 
   alpine-test-arm64:
     name: Alpine Test ARM64