interesting, i appreciate the correction. for some reason the google books date range searches linked from the n-grams page weren't loading for me, and the corresponding internet archive searches are also broken

there also seem to be some search hits from 'unit tests' for education, in which the 'unit' is presumably a course or subdivision thereof; https://www.google.com.ar/books/edition/The_Organization_of_... is an example, though sadly google refuses to show me any excerpts, perhaps because i'm in a country they don't like

https://www.google.com.ar/books/edition/Naval_Training_Bulle... does show an excerpt

so i guess the engineering use might date from later, perhaps as late as the 01960s