Atm program xUnit unit test
Is your mock ATM class literally named _atm?!  Or is it a non-static class that you have assigned to an instance named _atm?  Either way, I don't like it and find the _atm class to be useless in your limited example.
The BankAccount class, which you do not share, should have a Deposit and Withdraw method, both accepting a BankTransaction as a input argument.  You should include validation of a particular transaction within each method, i.e. do not accept a deposit of negative amount.  You also do not share BankTransaction with us.
CheckBalance is not needed, as you would just as easily show the BankAccount.AccountBalance property.  Also the property name could be shortened to Balance since it is the balance of the bank account.