Re-enable the GetLedgerDB action in the ChainDB QSM tests

[jorisdral]

Hmm, unfortunately changing the tests to work with the new LedgerDB went under the radar :P

When designing the LedgerDB API, the getLedgerDB function was removed from the ChainDB API, because LedgerDB was no longer a pure datastructure. It was replaced by more concrete functions, amongst which: getCurrentLedger, getImmutableLedger, getPastLedger, getHeaderStateHistory, getReadOnlyForkerAtPoint, getLedgerTablesAtFor, getStatistics.

Options I can think of for how to restore testing:

• Test (a subset of) the concrete functions by creating a command for each.
• Don't test the concrete functions, but use TestInternals to output some pure information about the LedgerDB. Maybe we could reconstruct a pure sequence of ledger states from the concrete LedgerDB impls (reading tables from disk too), and compare those against the model?

And no, there is no issue for it, but there probably should be

Originally posted by @jorisdral in #1267 (comment)

The following comment is also relevant: #1267 (comment)