bugfix: fix unittest error introduced in #2056 (#2136)

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## 📌 Description

In #2056 , when `world_size > available_gpus`, we should skip UT instead
of raise error.

cc @wenscarl for viz.

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## Summary by CodeRabbit

* **Tests**
* Enhanced test execution logic to improve reliability when handling
resource constraints.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

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Author: yzh119

tests/comm/test_trtllm_mnnvl_allreduce_custom_comm.py

@@ -250,7 +250,7 @@ def test_mnnvl_allreduce_custom_communicator(
 
     available_gpus = torch.cuda.device_count()
     if world_size > available_gpus:
-        raise ValueError(
+        pytest.skip(
             f"world_size {world_size} is greater than available_gpus {available_gpus}"
         )
     print(f"Running test for world_size={world_size}")