Translation:They had not tested the toy yet.
In every entry so far in the lesson, the verb at the end of the sentence has ended in "-en"; gegessen, verlassen, erfahren, why is this in the "-t" form?
The past participle can end in -t or in -en in German -- roughly speaking in -t for weak verbs (where the vowel does not change in the past tense) and -en for strong verbs (where the vowel does change).
Compare English, which also has past participles in both -ed (has looked, has used, has opened, has closed, ...) and -en (has given, has taken, has seen, has eaten, ...).
There is no separate "-en" form or "-t/-ed" form, both of which can apply to a given verb; they're all past participles and each verb will take one or the other.
I think it's not a particularly good translation.
prüfen is basically to examine or check. "verify" has an additional connotation that you have a certain quality in mind before the examination and that the examination has determined that the item does indeed have the quality you hoped or expected it would have.