[tests] migrate lora tests for pipelines to use new mixins (#14268)

* start migrating lora tests.

* massive refactor.

* up

* remove planning doc

* address review: tighten lora bias tests

* move run_pipe()

* collapse.

* up

* done

* update

* only use diffusers api contract in tests.

* fix: pass tmp_path through AuraFlow test_lora_B_bias override

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>