Filings UI: refactor Filing History List unit tests

This is a follow-on ticket for #15692.

Thanks to the refactoring above, individual ledger filings (including their differences) can be unit tested failrly easily. However, some previous unit tests were broken and were not fixed due to their scale.

THIS ticket is to clean up those old tests and write new unit tests to verify indidivual filings. This is basically a re-write of all FilingsHistoryList unit tests (with the advantage of lots of existing test code that can be reused).