[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/kernels/test_activation.py

@@ -2,11 +2,12 @@ from typing import Type
 
 import pytest
 import torch
-from allclose_default import get_default_atol, get_default_rtol
 
 from vllm.model_executor.layers.activation import (FastGELU, GeluAndMul,
                                                    NewGELU, SiluAndMul)
 
+from .allclose_default import get_default_atol, get_default_rtol
+
 DTYPES = [torch.half, torch.bfloat16, torch.float]
 NUM_TOKENS = [7, 83, 2048]  # Arbitrary values for testing
 D = [512, 4096, 5120, 13824]  # Arbitrary values for testing