add test engine defines on linux for imgui

cmake/imgui.cmake

@@ -40,4 +40,8 @@ set(
   ${IMGUI_PATH}/misc/cpp/imgui_stdlib.cpp
   ${IMGUI_FILE_DLG_PATH}/ImGuiFileDialog.cpp
   ${IMGUI_COLOR_TEXT_EDIT_PATH}/TextEditor.cpp
-)
+)
+
+if (LINUX)
+  add_definitions("-DIMGUI_ENABLE_TEST_ENGINE -DIMGUI_TEST_ENGINE_ENABLE_COROUTINE_STDTHREAD_IMPL=1")
+endif()