[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/tensorizer_loader/test_tensorizer.py

@@ -9,12 +9,13 @@ import pytest
 import ray
 import torch
 
-from tests.entrypoints.test_openai_server import ServerRunner
 from vllm import SamplingParams
 from vllm.model_executor.model_loader.tensorizer import (
     EncryptionParams, TensorizerConfig, TensorSerializer,
     is_vllm_serialized_tensorizer, load_with_tensorizer, open_stream)
 
+from ..utils import ServerRunner
+
 prompts = [
     "Hello, my name is",
     "The president of the United States is",