[inductor] Move fx-fusion tests to a separate file (#97028)
They're sort of independent of the rest of inductor, and this makes
them a bit easier to find and marginally faster to run.
Differential Revision: [D44168337](https://our.internmc.facebook.com/intern/diff/D44168337/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D44168337/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97028
Approved by: https://github.com/jansel
