[CI] Run v1/test_serial_utils.py in CI (vllm-project#16996)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

Author: russellb

.buildkite/test-pipeline.yaml

@@ -209,6 +209,7 @@ steps:
     - pytest -v -s v1/worker
     - pytest -v -s v1/structured_output
     - pytest -v -s v1/spec_decode
+    - pytest -v -s v1/test_serial_utils.py
     - pytest -v -s v1/test_stats.py
     - pytest -v -s v1/test_utils.py
     - pytest -v -s v1/test_oracle.py