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

[AswaniSahoo]

Summary

Adds documentation to the Testing section of CONTRIBUTING.md showing how to build C++ tests manually with CMake and run individual tests using ctest. This covers a practical workflow that is useful for attaching a debugger, running a single test in isolation, and keeping a build directory around for quick rebuild cycles.

The CMake invocation and flags are consistent with what test/run_oss_cpp_tests.sh already uses internally.

Fixes #11979

Test plan

• Content is accurate — CMake flags match test/run_oss_cpp_tests.sh
• Only modifies the "Running Tests Locally" section; no other sections changed
• Docs build succeeds (sphinx-build -b html docs/source docs/_build/html)
• git diff --check passes (no trailing whitespace)

cc @GregoryComer @AlannaBurke @mergennachin

[pytorch-bot]

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[Copilot]

Pull request overview

Adds contributor documentation for a manual CMake-based workflow to build and run ExecuTorch C++ tests locally, complementing the existing test/run_oss_cpp_tests.sh helper and enabling more iterative workflows (e.g., keeping a persistent build dir, running a single test via ctest).

Changes:

• Document a manual CMake configure/build/install flow for C++ tests under cmake-out.
• Document using ctest to list tests and run tests by regex from the build directory.
• Add brief guidance on extending the configuration with additional -DEXECUTORCH_BUILD_* options as needed.

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[AswaniSahoo]

@pytorchbot label "release notes: none"

[Copilot]

Pull request overview

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