[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/engine/output_processor/test_multi_step.py

@@ -4,7 +4,6 @@ from unittest.mock import MagicMock
 import pytest
 from transformers import PreTrainedTokenizer
 
-from tests.core.utils import create_seq_group
 from vllm.core.scheduler import Scheduler
 from vllm.engine.output_processor.multi_step import MultiStepOutputProcessor
 from vllm.engine.output_processor.stop_checker import StopChecker
@@ -14,6 +13,8 @@ from vllm.sequence import (CompletionSequenceGroupOutput, Logprob,
 from vllm.transformers_utils.detokenizer import Detokenizer
 from vllm.utils import Counter
 
+from ...core.utils import create_seq_group
+
 
 @pytest.mark.parametrize("seq_output_len", [128])
 @pytest.mark.parametrize("num_new_tokens", [1, 12])