Reset Dynamo Before Export for all XNNPACK Tests (#9561)

https://github.com/pytorch/executorch/actions/runs/14047575373/job/39331644423

There seems to be some CI issues with:
```
torch._dynamo.exc.FailOnRecompileLimitHit: recompile_limit reached with one_graph=True. Excessive recompilations can degrade performance due to the compilation overhead of each recompilation. To monitor recompilations, enable TORCH_LOGS=recompiles. If recompilations are expected, consider increasing
```

To help resolve this we reset dynamo at setup for all unittests. Let's
see if this helps