Decouple UTs between Framework and Plugins packages

### What you would like to be added?

As I mentioned in https://github.com/kubeflow/trainer/blob/3ec8f0705f515269b5ab8744c20b9d085f50d1ce/pkg/runtime/framework/core/framework_test.go#L51-L53, it would be better to decouple those UTs.

- [x] PlainML: #2469
- [x] Torch: #2492
- [ ] CoScheduler: https://github.com/kubeflow/trainer/pull/2582
- [x] MPI: #2481
- [ ] JobSet
- [ ] Replace actual runtime with fake runtime in https://github.com/kubeflow/trainer/blob/3ec8f0705f515269b5ab8744c20b9d085f50d1ce/pkg/runtime/framework/core/framework_test.go

### Why is this needed?

UTs should be independent as much as possible. The decoupled UTs make projects stable and allow us to easily find bug fixes and improve project development cycles.

### Love this feature?

Give it a 👍 We prioritize the features with most 👍