Import APIs don't work with DataCite test DOIs

[qqmyers]

Discovered during @kcondon's testing of #7504: The test in ImportDataCommand, which is intended to see if a DataCite DOI exists, always fails for test DataCite DOIs now (since their change from using the open 10.5072 test authority and requiring test accounts. As of that change, test DOIs are not indexed even when findable and thus URLs of the form https://doi.org/10.33564/FK27U7YBV always return 404.)
This affects the new migrate API in #7504 and should also affect the existing Dataverse APIs /api/dataverses/<id>/datasets/:import and /api/dataverses/<id>/datasets/:importddi APIs.

Since the intent of the test is just to verify that the DOI exists at DataCite, calling DOIDataCiteServiceBean.alreadyExists() for the DOI instead should work.

Note that this is a testing-only issue.