test : make the FA V-is-view-of-K case a test case parameter (#27394)

Resolve the TODO in test_flash_attn_ext: the branch that creates V as a
sub-view of K (MLA-based models) was hardcoded for the 576/512 head shapes.
Add a v_is_view_of_k test case parameter (default false) and select the
sub-view branch on it; the existing 576/512 (DeepSeek MLA) cases now pass it
explicitly, so the test coverage is unchanged.

Also add more V-is-sub-view-of-K cases: the 320/256 (Mistral4 MLA) and
192/128 head shapes, and full views with equal head sizes (128/128 F16,
64/64 q8_0).

Assisted-by: pi:llama.cpp/Qwen3.8-27B