Update download diff format tests

[DN6]

What does this PR do?

These tests download the v1-5-pruned-emaonly.ckpt for SD 1.5, however we've updated our torch.load to default to use the weights_only=True which is causing issues with this checkpoint since it can't load the serialized PT lightning callbacks. Updating the tests to use the safetensors files instead.

Fixes # (issue)

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