[CI/Build] Move test_utils.py to tests/utils.py (#4425)

Since #4335 was merged, I've noticed that the definition of ServerRunner in the tests is the same as in the test for OpenAI API. I have moved the class to the test utilities to avoid code duplication. (Although it only has been repeated twice so far, I will add another similar test suite in #4200 which would duplicate the code a third time)

Also, I have moved the test utilities file (test_utils.py) to under the test directory (tests/utils.py), since none of its code is actually used in the main package. Note that I have added __init__.py to each test subpackage and updated the ray.init() call in the test utilities file in order to relative import tests/utils.py.

tests/spec_decode/e2e/conftest.py

@@ -9,7 +9,6 @@ import torch
 from pynvml import (nvmlDeviceGetHandleByIndex, nvmlDeviceGetMemoryInfo,
                     nvmlInit)
 
-from tests.conftest import cleanup
 from vllm import LLM
 from vllm.engine.arg_utils import AsyncEngineArgs
 from vllm.engine.async_llm_engine import AsyncLLMEngine
@@ -21,6 +20,8 @@ from vllm.sequence import Logprob, MultiModalData
 from vllm.usage.usage_lib import UsageContext
 from vllm.utils import Counter, random_uuid
 
+from ...conftest import cleanup
+
 
 class AsyncLLM:
     """AsyncLLM