Refactor some testing helpers for FX graph cache testing (#121520)
Summary: I plan to enable the FX graph cache for more inductor unit tests. This PR does some refactoring to prepare by moving the `TestCase` base class to `torch._inductor.test_case` (which mirrors the existing `torch._dynamo.test_case`). In a subsequent diff, I'll modify tests importing `torch._dynamo.test_case.TestCase` to use `torch._inductor.test_case.TestCase` instead.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121520
Approved by: https://github.com/eellison
