api/add-subscriptions: Test fixture for "already_subscribed".

In templates/zerver/api/add-subscriptions, we have a sample fixture
for when the user being subscribed is already subscribed to a
stream. This commit tests that fixture against a running server.

templates/zerver/api/add-subscriptions.md

@@ -105,21 +105,7 @@ A typical successful JSON response may look like:
 A typical successful JSON response when the user is already subscribed to
 the streams specified:
 
-```
+{generate_code_example|add-subscriptions|fixture(already_subscribed)}
-{
-    'subscribed':{
-
-    },
-    'msg':'',
-    'result':'success',
-    'already_subscribed':{
-        'sample-bot@localhost':[
-            'Nonexistent',
-            'Verona'
-        ]
-    }
-}
-```
 
 A typical response for when the requesting user does not have access to
 a private stream and `authorization_errors_fatal` is `True`:

templates/zerver/api/fixtures.json

@@ -177,8 +177,20 @@
                     "new stream"
                 ]
             }
+        },
+        "already_subscribed": {
+            "already_subscribed":{
+                "newbie@zulip.com":[
+                    "new stream"
+                ]
+            },
+            "msg":"",
+            "result":"success",
+            "subscribed":{
+
             }
         },
+    },
     "remove-subscriptions": {
         "msg":"",
         "not_subscribed":[

zerver/lib/api_test_helpers.py

@@ -47,6 +47,18 @@ def add_subscriptions(client):
     assert result['result'] == 'success'
     assert 'newbie@zulip.com' in result['subscribed']
 
+def test_add_subscriptions_already_subscribed(client):
+    # type: (Client) -> None
+    result = client.add_subscriptions(
+        streams=[
+            {'name': 'new stream', 'description': 'New stream for testing'}
+        ],
+        principals=['newbie@zulip.com']
+    )
+
+    fixture = FIXTURES['add-subscriptions']['already_subscribed']
+    test_against_fixture(result, fixture)
+
 def create_user(client):
     # type: (Client) -> None
 
@@ -465,6 +477,7 @@ def test_streams(client):
     # type: (Client) -> None
 
     add_subscriptions(client)
+    test_add_subscriptions_already_subscribed(client)
     list_subscriptions(client)
     get_stream_id(client)
     get_streams(client)