make test module hook use save/load (#18284)

make test module hook use save/load (#18284)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18284
ghimport-source-id: 5a92c03fda19072ffb6afd40e0f56806716c7be6

Stack from [ghstack](https://github.com/ezyang/ghstack):
* #18296 [jit] Add namespacing for ScriptClasses
* **#18284 [jit] make test module hook use save/load**
* #18211 [jit] Turn script_type_parser into a class
* #18148 [jit] python interop for script classes

Instead of python-printing and comparing strings (which does not capture
depdency information, etc.), use save/load on in-memory buffers and
compare the main module contents inside the buffer

Reviewed By: ailzhang

Differential Revision: D14581129

fbshipit-source-id: 52264ae9ce076775ab3fd1a0c32c8d6f6677a903