Fixed randomly failing subscription test

The issue was that
```python
	 response = client.post(
            f'/api/feedback/', {
                "score": 23,
                "of_submission": response.data['submission']['pk'],
                "feedback_lines": {
                    2: {"text": "< some string >"},
                    3: {"text": "< some string >"}
                }
            }
        )
        self.assertEqual(status.HTTP_201_CREATED, response.status_code)
```
would sometimes get the one submission that only had 2 lines (since submission distribution hasn't been randomized for long, this issue didn't come up earlier) and thus fail as it should.
I changed the commented line to 1 and 2 in the POST. This should fix the issue.

core/tests/test_subscription_assignment_service.py

@@ -315,8 +315,8 @@ class TestApiEndpoints(APITestCase):
                 "score": 23,
                 "of_submission": response.data['submission']['pk'],
                 "feedback_lines": {
-                    2: {"text": "< some string >"},
-                    3: {"text": "< some string >"}
+                    1: {"text": "< some string >"},
+                    2: {"text": "< some string >"}
                 }
             }
         )

[Jan Maximilian Michal]

Nice!