Problem of crashing command on "lightningd/lightningd --network=testnet --log-level=debug"
The issue seems to be contained in the error message. 

Could not locate RPC credentials. No authentication cookie could be found, and no rpcpassword is set in the configuration file (/home/lightning/.bitcoin/bitcoin.conf)


So, set rpcuser and rpcpassword appropriately in ~/.bitcoin/bitcoin.conf.