Skip UCCL nixlbench ASIO tests due to flakiness (#2055)

## What?
Skip UCCL nixlbench test due to frequent issues on teardown
Follow up for https://github.com/ai-dynamo/nixl/pull/2000

## Why?
See https://github.com/ai-dynamo/nixl/issues/1999

Summary from triage agent:

The UCCL backend crashes on teardown (the benchmark row printed and both
processes logged "Destroying Engine…/Engine destroyed", then task 0
segfaulted — a race/crash in UCCL engine cleanup, not a hang or
timeout). This is the known-flaky UCCL nixlbench issue
https://github.com/ai-dynamo/nixl/issues/1999. Commit
https://github.com/ai-dynamo/nixl/pull/2000 attempted to mitigate it but
only commented out the ETCD-based UCCL loop (test_nixlbench.sh lines
122-132); the ASIO-based UCCL loop (lines 95-103) remained active and is
what ran and crashed. The pairwise ucx-v1.22.x variant (stage 351)
passed, confirming the failure is a nondeterministic UCCL crash. (The
extensive pin_thread_to_numa/selectNICs ERROR/WARN lines are benign
noise from the CI node's NUMA topology, not the cause.)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Tests**
* Disabled the GPU-gated UCCL ASIO transfer benchmark tests due to CI
flakiness.
  * Added a follow-up reference to track re-enabling the tests.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Ovidiu Mara <ovidium@nvidia.com>