Move fuser to test_jit_fuser (#18590)

Move fuser to test_jit_fuser (#18590)

Summary:
Start of breaking up test_jit.py

New files will have the format test_jit_* so they are easily grepable but remain in the same directory so we don't have to go through multiple sources for imports.

I am adding a test that's expected to fail to be sure it's running.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18590

Reviewed By: wanchaol

Differential Revision: D14677094

Pulled By: eellison

fbshipit-source-id: 9782c6aa9525bb6f332fc75cfff004c83a417522