I don't really get your question...
If you mean an exe app that stores testnet btc, there are wallets that allow testnet btc mode like Electrum and Bitcoin core .
You can just swithch to testnet mode by launching the exe in a testnet mode,in bitcoin core current installation using the GUI, the testnet is also installed along with your standard bitcoin core separately by default.
If this seem not to be what you meant then elaborate more......
I know about those wallets i mean one that is dedicated to the testnet only the testnet.
If you don't mind Android wallet, check
https://play.google.com/store/apps/details?id=de.schildbach.wallet_test. Other than that, i don't know anyone else who bother create testnet-only version of their Bitcoin wallet sofrware.