ci: declare all oakengine handle types opaque for Qt metatypes; defer gtest discovery on Windows

- Linux GCC rejects QMetaType instantiation over incomplete types
  (static_assert(sizeof(T))); Q_DECLARE_OPAQUE_POINTER every opaque
  OakEngine* handle in the public headers so QList<OakEngineTask*> etc
  compile
- Windows: gtest discovery ran the fresh exe before the DLLs were next
  to it; use DISCOVERY_MODE PRE_TEST on WIN32 and copy the DLLs for
  ctest time

engine/include/oakengine/task.h

@@ -288,6 +288,7 @@ oakengine_task_save_get_project(OakEngineTask *task);
 
 #ifdef __cplusplus
 }
+Q_DECLARE_OPAQUE_POINTER(OakEngineTask *)
 #endif
 
 #endif /* OAKENGINE_TASK_H */