@BowsacNoodle I'm not sure about what they were specifically testing for, but there are lots of reasons to do it, and they still do sub critical tests today (youtube.com/watch?v=bGf4-ZOjyVY).

Biggest reason is to make sure it works. The hardest part with making one is the purity needed of the sample, too much or too little reactive material would make the whole thing a very expensive dud. So they were checking that it was inline with what they expected, and the specific core they were using is one that they more or less knew worked, probably to form a baseline.