For Mandiri e-Cash payment method there aren’t any test data available, but you can see how it works with the payment flow given below.
Mandiri e-Cash Payment Flow
The customer enters his email address, name and phone number.
The customer receives the details of the payment and proceeds to pay with Mandiri e-Cash.
The customer logs in to his account by entering his phone number and PIN and makes the payment.
Upon completion of the payment flow, the customer is redirected back to your ReturnURL.