[UT] add pcp&dcp UT for mla_cp (#4953)

### What this PR does / why we need it?
Add UT of mla_cp, which include:
- test_compute_prefill_context_with_dcp_pcp
- test_reorg_kvcache_with_dcp_pcp
- test_out_lse_reshape
- test_npu_attention_update_with_dcp_pcp
- test_attention_with_mask_and_nomask_with_dcp_pcp
- test_process_attn_out_lse_with_dcp_pcp
- test_forward_prefill_cp_with_dcp_pcp

- vLLM version: v0.12.0
- vLLM main:
vllm-project/vllm@ad32e3e

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Signed-off-by: F.Liu <[email protected]>
Co-authored-by: F.Liu <[email protected]>

Author: ader47