GPU: Split GPU backend and Draw manager unit tests

Both the draw manager and gpu backend used the same compilation
directive for enablement. This PR seperates them into
`WITH_GPU_DRAW_TESTS` for draw manager related tests and
`WITH_GPU_BACKEND_TESTS` for gpu backend related tests.

Pull Request: https://projects.blender.org/blender/blender/pulls/132018

tests/python/CMakeLists.txt

@@ -988,7 +988,7 @@ else()
 endif()
 
 
-if(WITH_GPU_DRAW_TESTS)
+if(WITH_GPU_RENDER_TESTS)
   if(NOT OPENIMAGEIO_TOOL)
     message(STATUS "Disabling OpenGL draw tests because OIIO oiiotool does not exist")
   elseif(NOT EXISTS "${TEST_SRC_DIR}/opengl")