Fix flappy tests
https://trello.com/c/kKbu5XKq/261-fix-failing-tests
The first commit hopefully fixes the flappy tests in test/system/billing_expenses_test.rb
and test/system/client_notifications_test.rb
Not sure yet about test/system/clients_filter_dropdowns_test.rb
and test/system/group_offers_test.rb
, with these it looks like the clicks aren't applied correctly, e.g. in this screenshot the "Freiwillige/n hinzufügen" button is still pressed: