If it's the KDP uploading process you're testing, from memory I think the book's internals can be done with a .docx file, so maybe just fill the body with placeholder text. You could then upload with a mock-up cover image, check it and then remove from publication. From memory it's a pretty quick process to remove it. I highly doubt there's a real person checking contents, so a book filled with nonsense is unlikely to be flagged in the way existing literature might be.
If it's the KDP uploading process you're testing, from memory I think the book's internals can be done with a .docx file, so maybe just fill the body with placeholder text. You could then upload with a mock-up cover image, check it and then remove from publication. From memory it's a pretty quick process to remove it. I highly doubt there's a real person checking contents, so a book filled with nonsense is unlikely to be flagged in the way existing literature might be.