How to generate a testnet address? Bitcoin addresses can be generated on this site https://www.bitaddress.org, but the test network needs m or n at the beginning of the address, where can those be generated？
The site that you reference itself allows you to create Testnet addresses: https://www.bitaddress.org/?testnet=true (Hat tip to Nick ODell)

On the other hand, you could simply use a wallet that supports testnet, see e.g. Running Bitcoin Core in testnet mode.