tests: Add `assert_url_serves_contents_of_file()` to `ZulipTestCase`

Add `assert_url_serves_contents_of_file()` in `ZulipTestCase` class
and deduplicate some codes.

Fixes #1276.

zerver/lib/test_classes.py

@@ -273,6 +273,12 @@ class ZulipTestCase(TestCase):
 
         return [subscription.user_profile for subscription in subscriptions]
 
+    def assert_url_serves_contents_of_file(self, url, result):
+        # type: (str, bytes) -> None
+        response = self.client_get(url)
+        data = b"".join(response.streaming_content)
+        self.assertEquals(result, data)
+
     def assert_json_success(self, result):
         # type: (HttpResponse) -> Dict[str, Any]
         """

zerver/tests/test_upload.py

@@ -74,9 +74,7 @@ class FileUploadTest(ZulipTestCase):
 
         # Files uploaded through the API should be accesible via the web client
         self.login("hamlet@zulip.com")
-        response = self.client_get(uri)
-        data = b"".join(response.streaming_content)
-        self.assertEqual(b"zulip!", data)
+        self.assert_url_serves_contents_of_file(uri, b"zulip!")
 
     def test_file_too_big_failure(self):
         # type: () -> None
@@ -171,9 +169,7 @@ class FileUploadTest(ZulipTestCase):
 
         # In the future, local file requests will follow the same style as S3
         # requests; they will be first authenthicated and redirected
-        response = self.client_get(uri)
-        data = b"".join(response.streaming_content)
-        self.assertEqual(b"zulip!", data)
+        self.assert_url_serves_contents_of_file(uri, b"zulip!")
 
         # check if DB has attachment marked as unclaimed
         entry = Attachment.objects.get(file_name='zulip.txt')