Followup: split integration_spec.rb to instance/non-instance/shared specs
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Follow up from discussion in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/170439#note_2184855101
Integration behavior was moved to `Integrations::Base::Integration` module but the specs remain unchanged. We should split the `spec/models/integration_spec.rb` to shared/instance/non-instance spec files, for easier testing.
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