### What this PR does / why we need it?
fix test_qwen3_moe_external_launcher_ep_tp2 by
wait_until_npu_memory_free
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
- vLLM version: v0.16.0
- vLLM main:
15d76f74e2
Signed-off-by: hfadzxy <starmoon_zhang@163.com>