[Bugfix][CI] ALiBi test case in xformers multi_query_kv_attention

[NickLucche]

I've been meaning to add a test case to address this TODO https://github.com/vllm-project/vllm/blob/main/tests/kernels/test_attention.py#L367 but I then realized the test would break due to a dim expansion of the attention bias happening here https://github.com/vllm-project/vllm/blob/main/vllm/attention/backends/xformers.py#L783.

Therefore, this PR adds a test case for xformers multi query+alibi bias and fixes the previously untested scenario.

[github-actions]

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