This is a proof of concept for gitlab-org/gitlab-ce#30196.
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.
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