OpenGL: Restructure tests similar to D3D12 layout

- Move old test files to new unit/integration structure
- Add OpenGL Test Fixture
- Update CMakeLists.txt for new layout
- Add OpenGL_Test_Restructuring_Plan.md

tests/RHI/OpenGL/integration/CMakeLists.txt

@@ -0,0 +1,11 @@
+cmake_minimum_required(VERSION 3.15)
+
+project(OpenGL_Integration)
+
+set(ENGINE_ROOT_DIR ${CMAKE_SOURCE_DIR}/engine)
+
+find_package(Python3 REQUIRED)
+
+enable_testing()
+
+message(STATUS "OpenGL integration tests placeholder - to be implemented in Phase 5")