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52a3a487 - refactor wan pipeline tests to the new mixin structure (#14224)

refactor wan pipeline tests to the new mixin structure (#14224) * refactor wan pipeline tests to the new mixin structure * use num_videos_per_prompt in wan pipeline test config * testing_utils import single line * remove empty slow test placeholder --------- Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>