ci: rewrite Connect integration tests using `with-connect` Action

[nealrichardson]

See posit-dev/connectapi#475 for a proof of concept using this new GitHub Action, which encapsulates the logic for spinning up a test Connect server with Docker. Jotting down some notes here as I read through the existing tests:

• The current setup creates 4 users but only uses 2 of them, and the second user is only used to verify behavior that Connect itself tests (that collaborators are authorized to edit content). So we can cover all of the relevant behavior for pins using the single user account that with-connect will bootstrap.
• Test helpers should be moved out of the package proper and into setup.R (though most of them can probably be deleted with this change).
• Is it useful to preserve the PTD option if the local test Connect server isn't running?

[github-actions]

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