Create tests for `AccountBalanceNotifier`

[ethanwlee]

• using the existing test suite as an example, create tests to stress-test AccountBalanceNotifier
• the AccountBalanceNotifier widget is responsible for:
  • polling PFIs for all the account balances and summing up the balances into a single total and currency

[victoreronmosele]

.take

[github-actions]

Thanks for taking this issue! Let us know if you have any questions!