### What this PR does / why we need it?
Avoid oom during CI by using `with VllmRunner` instead of `LLM()`, and
enable `test_ngram_correctness`
### How was this patch tested?
CI passed.
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c
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Signed-off-by: fluctlux <38945811+fluctlux@users.noreply.github.com>
Co-authored-by: wangxiyuan <wangxiyuan1007@gmail.com>