build: move GTest and Google Benchmark setup into IcebergThirdpartyToolchain

[LuciferYang]

What

Fixes #829. The GoogleTest and Google Benchmark FetchContent setup lived in src/iceberg/test/CMakeLists.txt and src/iceberg/benchmark/CMakeLists.txt, apart from the other third-party dependencies. This follows the review discussion on #825, which suggested keeping dependency resolution in one place.

How

• Add resolve_gtest_dependency() and resolve_benchmark_dependency() to cmake_modules/IcebergThirdpartyToolchain.cmake, next to the existing resolve_* functions.
• Gate them on ICEBERG_BUILD_TESTS and ICEBERG_BUILD_BENCHMARKS, the same conditions that already guard the test and benchmark subdirectories.
• Both functions call prepare_fetchcontent() like the other resolvers, so a command-line -DCMAKE_COMPILE_WARNING_AS_ERROR=ON does not build the vendored GTest/Benchmark sources with -Werror.
• Version pins are unchanged. Only CMake is affected; Meson resolves these through subprojects/*.wrap, so it needs no change.

Verified

• -DICEBERG_BUILD_BENCHMARKS=ON: both dependencies fetch; benchmark_smoke and schema_test build; ctest -R schema_test passes.
• Tests and benchmarks both off: neither dependency is fetched.
• -DCMAKE_COMPILE_WARNING_AS_ERROR=ON with benchmarks on: configure and build succeed.
• pre-commit run -a clean.

---

This contribution was authored with assistance from Claude Opus 4.8, in line with the Iceberg guidelines for AI-assisted contributions (https://iceberg.apache.org/contribute/#guidelines-for-ai-assisted-contributions). All changes were reviewed and tested by the author.

[Copilot]

Pull request overview

This PR centralizes GoogleTest and Google Benchmark dependency resolution in cmake_modules/IcebergThirdpartyToolchain.cmake, aligning test/benchmark third-party setup with the rest of the project’s CMake dependency management (per #829 / discussion from #825).

Changes:

• Moved GoogleTest FetchContent configuration out of src/iceberg/test/CMakeLists.txt into IcebergThirdpartyToolchain.cmake via resolve_gtest_dependency().
• Moved Google Benchmark FetchContent configuration out of src/iceberg/benchmark/CMakeLists.txt into IcebergThirdpartyToolchain.cmake via resolve_benchmark_dependency().
• Added gating so these build-only dependencies are only resolved when ICEBERG_BUILD_TESTS / ICEBERG_BUILD_BENCHMARKS are enabled.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/iceberg/test/CMakeLists.txt	Removes local GTest FetchContent setup; relies on toolchain resolver.
src/iceberg/benchmark/CMakeLists.txt	Removes local benchmark FetchContent setup; relies on toolchain resolver.
cmake_modules/IcebergThirdpartyToolchain.cmake	Adds resolve_gtest_dependency() / resolve_benchmark_dependency() and conditionally invokes them based on build options.

[LuciferYang]

Thank you @wgtmac