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......
Why?
Without a valid reason, no one will think about creating a dedicated testnet wallet for you for free.
If you're willing to pay (in bitcoins, not testnet coins), you can post it in the services board: /index.php?board=52.0
What's the difference in using Bitcoin Core: bitcoin-qt --testnet? That should work if you need the whole testnet blockchain and a wallet.
Otherwise, any other wallet that supports testnet will do.
The whole point of testnet is to test things that you can use in the real wallet later. Since it's open source, you can strip out anything related to the real wallet, but it's kinda pointless to do.
if it's just a convenience type of thing, you could create a .bat (if you're running on windows) or an .sh (if you're running on linux/mac) to start core, electrum, sparrow, ... in testnet mode.
Why would you make it complicated, if you can actually get it in an easy way. Suppose you get testnet.exe the way you want, I don't think it will have any better usability than bitcoin-qt/bitcoind -testnet can provide.
However, if you want to do a more complex experiment, which Bitcoin-qt might not be able to execute those commands, then I suggest you to use bitcoinjs-lib (in testnet network).
I don't know your intentions but running testnet mode is only dedicated to testnet there's no interference from your standard wallet, coins from testnet can't be spend in real life but if you still insist on the testnet only wallet you can get the source code and eradicate what you don't need just as it been advised by Loyce but you will need understanding on how the code works or you can also practice what as been listed above by members
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.
Okay, so the actual use-case is to use a separate fiat-value which most wallets do is just stamp the value of mainnet to testnet.
(which is reasonable considering testnet has no "real" price)
Without questioning the rationale to that (I saw your other posts), it's quite doable by dirty-editing the price ticker APIs set by your favorite wallet.
For example, in Electrum's "exchange_rate.py" script; editing any USD price class to an API that supports testnet should show you the testnet price when that ticker is selected.
For example; from Bitpay's mainnet API to coinpaprika's testnet API:
Then, by selecting Bitpay in the GUI's menu: "Tools->Preferences->Fiat", Fiat Currency: "USD" Source: "Bitpay"
It will show testnet price based from coinpaprika's API (link) instead (but will also apply to mainnet):
If you really want it, edit the mentioned code and compile it yourself: github.com/spesmilo/electrum/blob/master/README.md#creating-binaries
I run Electrum from source to test it as quickly as possible BTW, so I don't have a compiled binary.