build: Optimize CI with test splitting and 32-core runner

Split 5 monolithic test targets into 139 individual test binaries for
better ctest parallelism, and upgrade ubuntu-debug to 32-core runner
with tuned build settings.

Test target splits (CMakeLists.txt changes only):
- velox_exec_test -> 76 individual tests
- velox_exec_util_test -> 9 individual tests
- velox_aggregates_test -> 43 individual tests
- velox_cache_test -> 5 individual tests
- velox_serializer_test -> 5 individual tests

CI workflow changes (ubuntu-debug):
- Runner: 16-core -> 32-core (128 GB RAM)
- NUM_THREADS: 16 -> 32
- MAX_HIGH_MEM_JOBS: 4 -> 12 (128 GB / 6.8 GB per heavy TU)
- MAX_LINK_JOBS: 2 -> 12 (shared links use ~1 GB each)
- ctest: -j 8 -> -j 24 --timeout 900
- ulimit -n 65536 to prevent "Too many open files"

Adapters job: add -DVELOX_BUILD_SHARED=ON for faster linking.

Validated on 48-core test machine (throttled to 32 threads):
- Build: 26m 34s (cold, no ccache)
- Test: 17m 45s (631 tests, 625 passed)
- Total: 44m 28s vs estimated ~70 min current CI

BUILD_PLAN.md contains the full analysis and profiling data.

Author: kgpai

.github/workflows/linux-build-base.yml

@@ -156,6 +156,7 @@ jobs:
             "-DVELOX_ENABLE_ABFS=ON"
             "-DVELOX_ENABLE_WAVE=ON"
             "-DVELOX_MONO_LIBRARY=ON"
+            "-DVELOX_BUILD_SHARED=ON"
           )
           if [[ "${USE_CLANG}" = "true" ]]; then
              scripts/setup-centos9.sh install_clang15; export CC=/usr/bin/clang-15; export CXX=/usr/bin/clang++-15; CUDA_FLAGS="-ccbin /usr/lib64/llvm15/bin/clang++-15";
@@ -222,7 +223,7 @@ jobs:
           python ./scripts/checks/run-clang-tidy.py -p $BUILD_PATH --commit $RANGE $FILES
 
   ubuntu-debug:
-    runs-on: 16-core-ubuntu
+    runs-on: 32-core-ubuntu
     # prevent errors when forks ff their main branch
     if: ${{ github.repository == 'facebookincubator/velox' }}
     name: Ubuntu debug with resolve_dependency
@@ -263,7 +264,7 @@ jobs:
         env:
           VELOX_DEPENDENCY_SOURCE: BUNDLED
           ICU_SOURCE: SYSTEM
-          MAKEFLAGS: NUM_THREADS=16 MAX_HIGH_MEM_JOBS=4 MAX_LINK_JOBS=2
+          MAKEFLAGS: NUM_THREADS=32 MAX_HIGH_MEM_JOBS=12 MAX_LINK_JOBS=12
         run: |
           EXTRA_CMAKE_FLAGS=(
             "-DCMAKE_LINK_LIBRARIES_STRATEGY=REORDER_FREELY"
@@ -297,7 +298,8 @@ jobs:
 
       - name: Run Tests
         run: |
-          cd _build/debug && ctest -j 8 --output-on-failure --no-tests=error
+          ulimit -n 65536
+          cd _build/debug && ctest -j 24 --timeout 900 --output-on-failure --no-tests=error
 
   fedora-debug:
     runs-on: 16-core-ubuntu