Use `sign_in` instead of `login_as` when we're not testing login flow
This is a proof of concept for #30196 (closed).
The actual login procedure is well-tested by
spec/features/login_spec.rb
, and we don't gain anything by also
thoroughly testing it here, in our second-slowest feature spec. In fact,
it only slows us down!
So instead we use sign_in
from the Devise::Test::IntegrationHelpers
module, which just sets the current user at the Warden level. This
drastically reduces the "setup" phase of every test in this file. A
non-scientific test run saw this drop from 633 to 231 seconds.