Considering testnet coins are supposed to have ZERO value, a 51% attack is not only meaningless but also pointless. Tbtc coins are only needed to test code, once the testing is done no one should care about them and in the past were returned to their source for others to use.

The only folks who would get hurt are the yutz's that 'mine' it for-profit by selling/exchanging tbtc for other coins. Seeing their 'holdings' disrupted certainly would not break my heart