Fix unit tests for the frontend language part (#872)

test/lang/test_anthropic_backend.py

@@ -7,14 +7,11 @@ from sglang.test.test_programs import test_mt_bench, test_stream
 
 class TestAnthropicBackend(unittest.TestCase):
     backend = None
-    chat_backend = None
 
-    def setUp(self):
-        cls = type(self)
-
-        if cls.backend is None:
-            cls.backend = Anthropic("claude-3-haiku-20240307")
-            set_default_backend(cls.backend)
+    @classmethod
+    def setUpClass(cls):
+        cls.backend = Anthropic("claude-3-haiku-20240307")
+        set_default_backend(cls.backend)
 
     def test_mt_bench(self):
         test_mt_bench()
@@ -30,5 +27,5 @@ if __name__ == "__main__":
 
     # global_config.verbosity = 2
     # t = TestAnthropicBackend()
-    # t.setUp()
+    # t.setUpClass()
     # t.test_mt_bench()