api/update-message: Make code examples and fixtures testable.

This commit uses the Markdown extension defined in
zerver/lib/bugdown/api_generate_examples to generate the
example fixture and code example, so that both are tested in
tools/lib/api_tests.

templates/zerver/api/fixtures.json

@@ -13,5 +13,9 @@
         "msg":"",
         "id":134,
         "result":"success"
+    },
+    "update-message": {
+        "msg":"",
+        "result":"success"
     }
 }

templates/zerver/api/update-message.md

@@ -27,21 +27,7 @@ curl -X "PATCH" {{ api_url }}/v1/messages/<msg_id> \
 
 <div data-language="python" markdown="1">
 
-```python
-#!/usr/bin/env python
-
-import zulip
-
-# Download ~/zuliprc-dev from your dev server
-client = zulip.Client(config_file="~/zuliprc-dev")
-
-# Edit a message
-print(client.update_message({
-    "message_id": 131,
-    "content": "New content"
-}))
-
-```
+{generate_code_example|update-message|method}
 
 </div>
 
@@ -89,12 +75,7 @@ You only have permission to edit a message if:
 
 A typical successful JSON response may look like:
 
-```
-{
-    'msg':'',
-    'result':'success'
-}
-```
+{generate_code_example|update-message|fixture}
 
 A typical JSON response for when one doesn't have the permission to
 edit a particular message:

zerver/lib/api_test_helpers.py

@@ -182,12 +182,20 @@ def update_message(client, message_id):
     # type: (Client, int) -> None
 
     assert int(message_id)
-    request = dict(
-        message_id=message_id,
-        content='new content',
-    )
+
+    # {code_example|start}
+    # Edit a message
+    # (make sure that message_id below is set to the ID of the
+    # message you wish to update)
+    request = {
+        "message_id": message_id,
+        "content": "New content"
+    }
     result = client.update_message(request)
-    assert result['result'] == 'success'
+    # {code_example|end}
+
+    fixture = FIXTURES['update-message']
+    test_against_fixture(result, fixture)
 
     # test it was actually updated
     url = 'messages/' + str(message_id)
@@ -196,12 +204,13 @@ def update_message(client, message_id):
         method='GET'
     )
     assert result['result'] == 'success'
-    assert result['raw_content'] == 'new content'
+    assert result['raw_content'] == request['content']
 
 TEST_FUNCTIONS = {
     'render-message': render_message,
     'stream-message': stream_message,
     'private-message': private_message,
+    'update-message': update_message,
 }
 
 # SETUP METHODS FOLLOW