* User must install Paddle python package before unittest.
* Or use docker to build Paddle

* Do not use `virtualenv` in Docker
* Split unittest into many target, make them parallel

and disable both when build doc and MacOS

`merge_static_libs` also have the similar logic of using ${dummyfile},
I am not sure if there needs a change or not.

* Refine register methods, make Op can get rid of whole-archieve
* `USE_OP` before a op is used.
* Add unittest for add_op.