A testnet blockchain and lightclient / thin-client are two different concepts all together and they can't be actually differentiated in general terms as such. But, anyways I'll attempt to explain to you what they both are:
A testnet is an alternative blockchain as compared to mainnet which is the main blockchain. The testnet is generally created for the purpose of testing proposals, new feature enhancements and probably soft forks to check for rates of success before implementing on the mainnet. This is done to ensure that transactions, security and performance are not affected on the mainnet as otherwise would be detrimental to investors funds if anything fails. Testnet in the case of Bitcoins consists of a unique address (Testnet 3) addresses and testnet coins which have no value.
A lightclient is an application that allows the users to run a wallet without having to have the entire blockchain downloaded locally to perform validations for transactions and mined blocks throughout the chain. A lightclient only downloads the blockchain headers from a bunch of servers that host the entire blockchain and some of such clients may only download block headers for the transactions and addresses that you have within your wallet. This save user's disk space and time. Such clients cannot validate blocks unlike fullnode clients or "thick" clients. A lightclient wallet can be made to run on a testnet or mainnet.
Thank you very for the explanation the main idea is understood now
