extern/googletest: Add testing library

Need a testing framework to test the IR layer. GoogleTest is a popular
choice so it was chosen.

Signed-off-by: Ronald Caesar <github43132@proton.me>

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "3rd_Party/imgui"]
 	path = 3rd_Party/imgui
 	url = https://github.com/ocornut/imgui.git
+[submodule "3rd_Party/googletest"]
+	path = 3rd_Party/googletest
+	url = https://github.com/google/googletest.git

3rd_Party/PINNED_DEPENDENCIES.md

@@ -32,3 +32,13 @@ This document tracks all pinned third-party submodules in the Pound project. Eac
 - **Last Review**: 2025-09-20
 - **Next Review**: 2026-03-20
 
+### GoogleTest
+- **Repository**: https://github.com/google/googletest
+- **Version Tag**: v1.17.0
+- **Commit Hash**: 52eb8108c5bdec04579160ae17225d66034bd723
+- **License**: BSD-3-Clause
+- **Purpose**: Provides the testing and mocking framework for Pound.
+- **Pinning Date**: 2025-11-09
+- **Pinning Reason**: Dependency added for the first time.
+- **Last Review**: 2025-11-09
+- **Next Review**: 2026-05-09

3rd_Party/googletest

@@ -0,0 +1 @@
+Subproject commit 52eb8108c5bdec04579160ae17225d66034bd723

CMakeLists.txt

@@ -80,6 +80,8 @@ foreach(line ${submodule_lines})
             verify_pinned_commit("imgui" "${submodule_path}" "${commit_hash}" "bf75bfec48fc00f532af8926130b70c0e26eb099")
         elseif(submodule_path STREQUAL "3rd_Party/SDL3")
             verify_pinned_commit("SDL3" "${submodule_path}" "${commit_hash}" "a96677bdf6b4acb84af4ec294e5f60a4e8cbbe03")
+        elseif(submodule_path STREQUAL "3rd_Party/googletest")
+            verify_pinned_commit("SDL3" "${submodule_path}" "${commit_hash}" "52eb8108c5bdec04579160ae17225d66034bd723")
         endif()
 
         message(STATUS "Verified submodule: ${submodule_path} (${commit_hash})")