[Speculative Decoding] Test refactor

[LiuXiaoxuanPKU]

Refactor Speculative Decoding tests to remove AsyncLLMEngine. Related to #8126.

[github-actions]

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[LiuXiaoxuanPKU]

/ready

[youkaichao]

thanks for the great simplification! the code looks much better now.

[youkaichao]

for the test failure I disabled, to reproduce and investigate, run the following in L4 machines:

docker run -it --gpus all --ipc=host -e HF_TOKEN -v ~/.cache/huggingface:/root/.cache/huggingface public.ecr.aws/q9t5s3a7/vllm-ci-test-repo:f3d79ad8513645851909fc9ea217a9ad0c413427
pytest -v -s tests/spec_decode/e2e/test_multistep_correctness.py -k "test_spec_decode_e2e_greedy_correctness_tiny_model_bs1 or test_spec_decode_e2e_with_detokenization"

I find the problem is the spec decode metrics are not collected.

vllm/vllm/engine/llm_engine.py

Lines 1900 to 1905 in 7015417

	if model_output and (model_output[0].spec_decode_worker_metrics
	is not None):
	spec_decode_metrics = model_output[0].spec_decode_worker_metrics
	else:
	spec_decode_metrics = None

after these lines, spec_decode_metrics is None.

cc @comaniac if you have bandwidth to investigate in the future.