slh-dsa: migrate to nextest, shard tests (#914)

GitHub CI is too slow and most of the time times out before we can run
all the tests.

This migrates the tests to using nextest to use their sharding
implementation.

.github/workflows/slh-dsa.yml

@@ -15,6 +15,8 @@ defaults:
 env:
   CARGO_INCREMENTAL: 0
   RUSTFLAGS: "-Dwarnings"
+  CARGO_TERM_COLOR: always
+  NEXTEST_NO_TESTS: warn
 
 jobs:
   no_std:
@@ -35,19 +37,76 @@ jobs:
           targets: ${{ matrix.target }}
       - run: cargo build --target ${{ matrix.target }} --no-default-features
 
-  test:
+  # because we're sharding tests, we'll compile them once first
+  # upload that as artifact, and then re-download those artifacts
+  # and run the tests.
+  # This is heavily insired by https://github.com/nextest-rs/reuse-build-partition-example/blob/main/.github/workflows/ci.yml
+  build-test:
     runs-on: ubuntu-latest
     strategy:
       matrix:
         rust:
           - 1.81.0 # MSRV
           - stable
+        test_config:
+          - --no-default-features
+          - --all-features
+          - default
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ matrix.rust }}
+      - uses: taiki-e/install-action@nextest
       - run: cargo check --all-features
-      - run: cargo test --no-default-features
-      - run: cargo test
-      - run: cargo test --all-features
+      - name: Build and archive tests
+        run: |
+          PROFILE="${{ matrix.test_config }}"
+          if [ "$PROFILE" = "default" ]; then
+            PROFILE="--features default"
+          fi
+          cargo nextest archive --archive-file nextest-archive-${{ matrix.rust }}-${{ matrix.test_config }}.tar.zst $PROFILE
+      - name: Upload archive to workflow
+        uses: actions/upload-artifact@v4
+        with:
+          name: nextest-archive-${{ matrix.rust }}-${{ matrix.test_config }}
+          # NOTE: upload-artifact does not respect the working directory, we need to prefix it.
+          # https://github.com/actions/upload-artifact/issues/294
+          path: slh-dsa/nextest-archive-${{ matrix.rust }}-${{ matrix.test_config }}.tar.zst
+
+          if-no-files-found: error
+          compression-level: 0
+          retention-days: 1
+
+  run-test:
+    name: slh-dsa/run tests
+    needs: build-test
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        test-partition: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]
+        rust:
+          - 1.81.0 # MSRV
+          - stable
+        test_config:
+          - --no-default-features
+          - --all-features
+          - "default"
+    steps:
+      - uses: actions/checkout@v4
+      - run: mkdir -p ~/.cargo/bin
+      - uses: taiki-e/install-action@nextest
+      - name: Download test archive
+        uses: actions/download-artifact@v4
+        with:
+          name: nextest-archive-${{ matrix.rust }}-${{ matrix.test_config }}
+          path: slh-dsa
+      - name: Run tests (${{ matrix.rust }}) (${{ matrix.test_config }})
+        run: |
+          PROFILE="${{ matrix.test_config }}"
+          if [ "$PROFILE" = "default" ]; then
+            PROFILE="--features default"
+          fi
+
+          ~/.cargo/bin/cargo-nextest nextest run --archive-file nextest-archive-${{ matrix.rust }}-${{ matrix.test_config }}.tar.zst \
+            --partition count:${{ matrix.test-partition }}/16