no one has ever bothered to do a spam attack (or the pretty word: stress test) dash or any other altcoin.
Are you kidding me? XRP, ETH, and many other coins have suffered spam attack multiple times before.
Bitcoin is special, but not so special as you claimed.
no they have not. maybe someone did a very short period of spam attack as a test for a couple of hours once, but none of them experienced any real spam attack.
and don't confuse a spam attack with a simple increase in number of transactions because price is pumping or dumping and people move their coins in a panic to sell them.