[Bugfix] fix sleepmode level2 e2e test (#4019)

### What this PR does / why we need it?

enable sleepmode level2 e2e test and add the check logic to ensure the
nz is not enabled.

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

use e2e tests


- vLLM version: v0.11.0
- vLLM main:
83f478bb19

Signed-off-by: wangx700 <wangxin700@huawei.com>

tests/e2e/singlecard/test_camem.py

@@ -18,6 +18,8 @@
 #
 
 import gc
+import os
+from unittest.mock import patch
 
 import torch
 from vllm import SamplingParams
@@ -71,6 +73,7 @@ def test_basic_camem():
 
 
 @fork_new_process_for_each_test
+@patch.dict(os.environ, {"VLLM_ASCEND_ENABLE_NZ": "0"})
 def test_end_to_end():
     free, total = torch.npu.mem_get_info()
     used_bytes_baseline = total - free  # in case other process is running