api/create-user: Test fixture for "email already in use" error.

In templates/zerver/api/create-user.md, we have a sample fixture
for when a client attempts to create a user with the same email
as an existing user. This commit adds a test for that fixture.

Note that this error payload is specific to client.create_user
and this error payload isn't generated anywhere else.

templates/zerver/api/create-user.md

@@ -72,15 +72,9 @@ zulip(config).then((client) => {
 
 A typical successful JSON response may look like:
 
-{generate_code_example|create-user|fixture}
-
+{generate_code_example|create-user|fixture(successful_response)}
 
 A typical JSON response for when another user with the same
 email address already exists in the realm:
 
-```
-{
-    'msg':"Email 'newbie@zulip.com' already in use",
-    'result':'error'
-}
-```
+{generate_code_example|create-user|fixture(email_already_used_error)}

templates/zerver/api/fixtures.json

@@ -149,8 +149,14 @@
         ]
     },
     "create-user": {
-        "msg":"",
-        "result":"success"
+        "successful_response": {
+            "msg":"",
+            "result":"success"
+        },
+        "email_already_used_error": {
+            "msg": "Email 'newbie@zulip.com' already in use",
+            "result": "error"
+        }
     },
     "get-profile": {
         "client_id":"74c768b081076fdb3c4326256c17467e",

zerver/lib/api_test_helpers.py

@@ -108,7 +108,13 @@ def create_user(client):
     result = client.create_user(request)
     # {code_example|end}
 
-    fixture = FIXTURES['create-user']
+    fixture = FIXTURES['create-user']['successful_response']
+    test_against_fixture(result, fixture)
+
+    # Test "Email already used error"
+    result = client.create_user(request)
+
+    fixture = FIXTURES['create-user']['email_already_used_error']
     test_against_fixture(result, fixture)
 
 def get_members(client):