test: require V0 in vllm test

Signed-off-by: Masataro Asai <guicho2.71828@gmail.com>

Author: guicho271828

test/backends/test_vllm.py

@@ -1,3 +1,4 @@
+import os
 import pydantic
 import pytest
 from typing_extensions import Annotated
@@ -38,6 +39,10 @@ def session(backend):
     yield session
     session.reset()
 
+
+def test_v0_api(session):
+    assert os.environ["VLLM_USE_V1"] == "0"
+
 @pytest.mark.qualitative
 def test_system_prompt(session):
     result = session.chat(