api docs: Test sample fixture for message edit permission error.

In templates/zerver/api/update-message.md, we have a sample fixture
for when a zulip.Client does not have the permission to update/edit
a particular message. This commit adds a test for that fixture.
Also, tools/test-api now also uses a non-admin client for this test,
which might come in handy in the future.

templates/zerver/api/fixtures.json

@@ -254,5 +254,10 @@
         "msg":"Missing 'content' argument",
         "result":"error",
         "var_name":"content"
+    },
+    "update-message-edit-permission-error": {
+        "code":"BAD_REQUEST",
+        "msg":"You don't have permission to edit this message",
+        "result":"error"
     }
 }

templates/zerver/api/update-message.md

@@ -80,10 +80,4 @@ A typical successful JSON response may look like:
 A typical JSON response for when one doesn't have the permission to
 edit a particular message:
 
-```
+{generate_code_example|update-message-edit-permission-error|fixture}
-{
-    'code':'BAD_REQUEST',
-    'result':'error',
-    'msg':"You don't have permission to edit this message"
-}
-```

tools/test-api

@@ -15,6 +15,7 @@ from zulip import Client
 
 from tools.lib.test_server import test_server_running
 from zerver.lib.api_test_helpers import test_the_api, test_invalid_api_key
+from zerver.lib.api_test_helpers import test_update_message_edit_permission_error
 
 os.environ['DJANGO_SETTINGS_MODULE'] = 'zproject.test_settings'
 django.setup()
@@ -35,6 +36,19 @@ with test_server_running(external_host='zulipdev.com:9981'):
     print("Running API tests...")
     test_the_api(client)
 
+    email = 'newbie@zulip.com'  # newbie is not an admin
+    realm = get_realm("zulip")
+    api_key = get_user(email, realm).api_key
+    site = 'http://zulip.zulipdev.com:9981'
+    nonadmin_client = Client(
+        email=email,
+        api_key=api_key,
+        site=site
+    )
+
+    # Run tests that require non-admin client
+    test_update_message_edit_permission_error(client, nonadmin_client)
+
     # Test error payloads
     client = Client(
         email=email,

zerver/lib/api_test_helpers.py

@@ -331,6 +331,25 @@ def update_message(client, message_id):
     assert result['result'] == 'success'
     assert result['raw_content'] == request['content']
 
+def test_update_message_edit_permission_error(client, nonadmin_client):
+    # type: (Client, Client) -> None
+    request = {
+        "type": "stream",
+        "to": "Denmark",
+        "subject": "Castle",
+        "content": "Something is rotten in the state of Denmark."
+    }
+    result = client.send_message(request)
+
+    request = {
+        "message_id": result["id"],
+        "content": "New content"
+    }
+    result = nonadmin_client.update_message(request)
+
+    fixture = FIXTURES['update-message-edit-permission-error']
+    test_against_fixture(result, fixture)
+
 def register_queue(client):
     # type: (Client) -> str