Salmen did you know coinb.in has testnet support:
https://coinb.in/#settingsYou can test hodl addresses on testnet instead. Electrum has testnet support so you can use that as well.
For Bitcoin testnet vast of the functions on Coinb.in are constrained and Electrum does not support redeeming time-locked address.
hodl addresses work on testnet. i speak from experience using coinb.in
as for electrum i mention it because in order to generate a hodl address you'll need to supply the public key of one of your legacy addresses, acquire testnet bitcoins from a faucet and then send those to the hodl address. for all that you can use an electrum testnet wallet. you're still going to have to use coinb.in to sign the transaction spending from the hodl address.