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.

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