[JIT] Move TestBackend to test directory (#40840)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40840
**Summary**
This commit moves the TestBackend used for the JIT backend
extension to the tests directory. It was temporarily placed
in the source directory while figuring out some details of
the user experience for this feature.
**Test Plan**
`python test/test_jit.py TestBackends`
**Fixes**
This commit fixes #40067.
Test Plan: Imported from OSS
Differential Revision: D22418682
Pulled By: SplitInfinity
fbshipit-source-id: 9356af1341ec4d552a41c2a8929b327bc8b56057
