/usr/share/cups/data/testprint is in cups-filters 1.0.18-0ubuntu0.4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Template default-testpage.pdf
Show printer-name printer-info printer-location printer-make-and-model printer-driver-name printer-driver-version paper-size imageable-area job-id options time-at-creation time-at-processing
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