The test step should not use the same metrics objects as validation

[carmocca]

🐛 Bug

https://github.com/PyTorchLightning/lightning-flash/blob/eb1fcd3925ee297d08704d48c5fc57624665ad96/flash/core/model.py#L424

[ethanwharris]

We do a deepcopy of the metrics object here: https://github.com/PyTorchLightning/lightning-flash/blob/eb1fcd3925ee297d08704d48c5fc57624665ad96/flash/core/model.py#L335 We could add something to do the same for test too. Simple test for correctness here that should be expanded to include val and test: https://github.com/PyTorchLightning/lightning-flash/blob/eb1fcd3925ee297d08704d48c5fc57624665ad96/tests/core/test_model.py#L345