@noshch21 Thank you for taking time to read my article. In simple words testnet is a separate block chain used by devolopers on any cryptocurrency to test any changes prior to it's launch to public. The cryptocurrency in testnet holds value less than the actual cryptocurrency. So their are less miner doing this type of mining. Anyone can mine those coins but they hold less value.
RE: Metropolis ahead: Ethereum developers Set September Date for Testnet release.