tests: Switch test_i18n fetch helper to use client_foo methods.

38f2a2f475 updated the comment but not the code.  Using
`self.client.post` instead of `self.client_post` means that we do not
set the host headers correctly.

zerver/tests/test_i18n.py

@@ -90,7 +90,7 @@ class TranslationTestCase(ZulipTestCase):
     def fetch(
         self, method: str, url: str, expected_status: int, **kwargs: Any
     ) -> "TestHttpResponse":
-        response = getattr(self.client, method)(url, **kwargs)
+        response = getattr(self, f"client_{method}")(url, **kwargs)
         self.assertEqual(
             response.status_code,
             expected_status,