investigate and fix cmd unit tests

[acharb]

some of the unit tests in cmd are failing, but I'm not sure if it's code we are still using. eg. TestExportChanges/changes_from_single_ledger using export_ledger_entry_changes (are we using this command anywhere?). Investigate if these command are something we still need and if so fix.

Once the cmd unit testing works we can add to the github workflow for automated testing (pr for reference)

[sydneynotthecity]

Fixed the export_transactions and export_operations tests. The other broken tests require a working captive core, which i'm still working on.

[sydneynotthecity]

@amishas157 we can close this ticket, correct? I believe #274 resolved

[amishas157]

Yes., the docs for running tests are available in https://github.com/stellar/stellar-etl?tab=readme-ov-file#running-tests