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_index.py

@@ -2,11 +2,13 @@ import pytest
 from flask import url_for
 
 
-def test_logged_in_user_redirects_to_choose_service(app_,
-                                                    api_user_active,
-                                                    mock_get_user,
-                                                    mock_get_user_by_email,
-                                                    mock_login):
+def test_logged_in_user_redirects_to_choose_service(
+    app_,
+    api_user_active,
+    mock_get_user,
+    mock_get_user_by_email,
+    mock_login,
+):
     with app_.test_request_context():
         with app_.test_client() as client:
             client.login(api_user_active)