[CI/Test] improve robustness of test (vllm_runner) (#5357)

[CI/Test] improve robustness of test by replacing del with context manager (vllm_runner) (#5357)

tests/engine/test_stop_strings.py

@@ -10,7 +10,8 @@ MAX_TOKENS = 200
 
 @pytest.fixture(scope="session")
 def vllm_model(vllm_runner):
-    return vllm_runner(MODEL)
+    with vllm_runner(MODEL) as vllm_model:
+        yield vllm_model
 
 
 @pytest.mark.skip_global_cleanup