ci: run tests with single thread to avoid HDF5 cleanup issues

HDF5 library has known cleanup issues when tests run in parallel,
causing 'infinite loop closing library' and SIGSEGV on Linux.
This is a workaround until the root cause is investigated.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: shinaoka

.github/workflows/ci.yml

@@ -89,7 +89,8 @@ jobs:
         run: cargo build --workspace --verbose
       - name: Run tests
         shell: bash -el {0}
-        run: cargo test --workspace --verbose
+        # Run with single thread to avoid HDF5 library cleanup issues
+        run: cargo test --workspace --verbose -- --test-threads=1
 
   test-features:
     name: test features (${{ matrix.features }})
@@ -123,7 +124,7 @@ jobs:
         run: cargo build --workspace --features "${{ matrix.features }}" --verbose
       - name: Test with features
         shell: bash -el {0}
-        run: cargo test --workspace --features "${{ matrix.features }}" --verbose
+        run: cargo test --workspace --features "${{ matrix.features }}" --verbose -- --test-threads=1
 
   macos:
     name: macOS
@@ -140,7 +141,7 @@ jobs:
           HDF5_PREFIX=$(brew --prefix hdf5)
           echo "HDF5_DIR=${HDF5_PREFIX}" >> $GITHUB_ENV
       - name: Build and test
-        run: cargo test --workspace --verbose
+        run: cargo test --workspace --verbose -- --test-threads=1
 
   interop-julia:
     name: Julia interop