give state dict creation its own method in IP-adapter pipeline tests

For models we use a method in the test Mixin that the models can override to generate the necessary state dict. Let's follow that convention here as well

https://github.com/huggingface/diffusers/blob/a8c33fe99f5603c907e6722325e4a68e3717b62b/tests/models/testing_utils/ip_adapter.py#L76

Override in the specific model class:
https://github.com/huggingface/diffusers/blob/a8c33fe99f5603c907e6722325e4a68e3717b62b/tests/models/transformers/test_models_transformer_flux.py#L322

_Originally posted by @DN6 in https://github.com/huggingface/diffusers/pull/14622#discussion_r3878211632_