Check flash messages in some system tests

This way it's more clear what the user experience is during the process.

Note this did not result in flaky tests, since the tests continue by
clicking links, that are only present after the request has finished.
However, we're adding expectations so we don't have to think whether the
tests could be flaky and because this way we're also testing the user
experience; it would be strange for a user if they were redirected to a
page without a flash message.

spec/system/admin/budgets_wizard/groups_spec.rb

@@ -103,6 +103,8 @@ describe "Budgets wizard, groups step", :admin do
       fill_in "Group name", with: "Group wiht typo"
       click_button "Continue to headings"
 
+      expect(page).to have_content "Group created successfully"
+
       click_link "Go back to edit group"
 
       expect(page).to have_field "Group name", with: "Group wiht typo"