Event Registration Test-drive: Bypass check for existing registration from this contact

Rationale
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Site administrators often need to do several test-drive registrations for an event to make sure things are working as planned. Currently, they can only do a single registration - and then they hit the "Oops already registered" message. The workaround is to do test-drives as anonymous - but this is pretty inconvenient, and may be impossible for events that are only open to authenticated users / have ACLs.

Implementation
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Modify event registration logic to skip the check on existing registration by this user / matching contact WHEN action=preview (e.g. test-drive mode).