docs: document manual CMake build and ctest workflow for C++ tests

CONTRIBUTING.md

@@ -321,6 +321,25 @@ CI is run automatically on all pull requests. However, if you want to run tests
 - The `test/run_oss_cpp_tests.sh` script will build and run C++ tests locally
 - Running `pytest` from the root directory will run Python tests locally. Make sure to run this after finishing [Dev Install](#dev-install).
 
+To build C++ tests manually with CMake, run the following from the repository root:
+
+```bash
+cmake . -Bcmake-out -DCMAKE_INSTALL_PREFIX=cmake-out -DEXECUTORCH_BUILD_TESTS=ON
+cmake --build cmake-out -j9 --target install
+```
+
+You can then use `ctest` to list or run individual C++ tests directly:
+
+```bash
+ctest --test-dir cmake-out -N
+ctest --test-dir cmake-out -R <test_name_regex> --output-on-failure
+```
+
+This workflow is useful when you want to rerun one test, attach a debugger to a
+test binary under `cmake-out`, or keep a build directory around for quick rebuild
+cycles. Add the same `-DEXECUTORCH_BUILD_*` options used by
+`test/run_oss_cpp_tests.sh` when the test needs optional kernels or extensions.
+
 ### Writing Tests
 To help keep code quality high, ExecuTorch uses a combination of unit tests and
 end-to-end (e2e) tests. If you add a new feature or fix a bug, please add tests