I generated bitcoin TESTNET paper wallet, but I do not know how to import it to testnet client(bitcoin core testnet).
For most purposes, testnet mode works just like production mode. You can import your private key using the importprivkey command.
How to import private keys:
https://en.bitcoin.it/wiki/How_to_import_private_keys#Using_bitcoindTestnet differences:
https://en.bitcoin.it/wiki/Testnet#Differences