Create tests for `TransactionNotifier`

[ethanwlee]

• using the existing test suite as an example, create tests to stress-test TransactionNotifier
• the TransactionNotifier widget is responsible for:
  • polling PFIs for a specific exchange id until an exchange is considered closed or until it is closed by the caller which started the polling

[victoreronmosele]

.take

[github-actions]

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