hi is there way to write test case with the mainnet deployed contract? Want to test with ston.fi 's contract

you can send transactions to any contract, contracts response regardless of your purpose of this transaction. You need TONCOIN to do it (testnet coins/mainnet coins).
so, clone contracts and write your tests locally using simulator. It will be much cheaper.

I want to simulator in test-cases

Try https://ide.nujan.io/ this

Do you have something similar but for func?

Yes, when you create a new project it will ask for Func or tact . choose one