Normalize whitespace in test arguments

We have a bunch of different styles of handling when function
definitions span multiple lines, which they almost always do with tests.

Here’s why an argument per line, single indent is best:
- cleaner diffs when you change the name of a method (one line change
  instead of multiple lines)
- works better on narrow screens, eg Github’s diff view, or with two
  terminals side by side on a laptop screen
- works with any editor’s indenting shortcuts, no need for an IDE

Also, trailing comma in the list of arguments is good because adding a
new argument to a method becomes a one line, not two line diff.

tests/app/main/views/test_letters.py

@@ -21,7 +21,7 @@ def test_letters_access_restricted(
     can_send_letters,
     response_code,
     mock_get_service_templates,
-    url
+    url,
 ):
     service = service_json(can_send_letters=can_send_letters)
     mocker.patch('app.service_api_client.get_service', return_value={"data": service})
@@ -39,7 +39,7 @@ def test_letters_lets_in_without_permission(
     mock_has_permissions,
     api_user_active,
     mock_get_service_templates,
-    url
+    url,
 ):
     service = service_json(can_send_letters=True)
     mocker.patch('app.service_api_client.get_service', return_value={"data": service})