I think I've seen 'dursn't' in books, occasionally

According to Google, Dickens. Though, maybe dasn't for Tom and Huck.

"Oh! I durstn't do it," rejoined the small servant; "Miss Sally 'ud kill me, if she know'd I come up here."

But I think these forms are from the present "dares not", whereas durst is a preterite or past participial form of to dare.

I durst not go abroad == I dared not go abroad.