First, consider using AutoText instead of an external file.
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Bookmarks can only refer to one place in a document. Consider using the StyleRef Field instead. That would require that your TestName be set in the Style referenced by the StyleRef field. When a new test is marked with that Style, the next StyleRef field will change, but not previous ones (ones before the page on which the Style appears. The Style can be a character style rather than a paragraph style.