How does this affect Bitcoin?
Is it possible to create a new testnet if they fork it?
It doesn't affect bitcoin at all, the testnet is separate and as the name indicates, for testing purposes only. It's like a replica network but with
useless worthless coins. Forking it will likely result to two chains being active, little more than an hour to find out what's going to happen.
No reason to create a new testnet though, this fork could be be temporary, there's no profit from throwing significant amounts of hash to a network with useless coins. In the event that two blockchains remain active at the same time services using the testnet will function normally under the non-fork blockchain.