You can signup for a Braintree Sandbox if you wanted to test transactions through Braintree. You can register here: https://www.braintreepayments.com/au/sandbox
Once registered you can follow the usual guide to obtain the Braintree details for placing them into a Jomablue Test event.
If you're looking for your account ID in the Sandbox you can find them by navigating to:
Account > Merchant Account Info
It will be listed in the table under Merchant Account ID column.
Adding Braintree Staging to Jomablue
When adding the Braintree details to Jomablue Payments page, ensure the Environment is set to "sandbox".
Once the Braintree Staging details are added you can now checkout of a Site using the test credit card details (providing all other Jomablue steps have been completed).
For more information see the Braintree Staging Articles:
Staging Credit Cards:
Error Codes and Transaction Amount Limits: