Remove in-memory testing modes

[SirTyson]

As of #4562, some tests cases require committing directly to the LedgerTxnRoot via an "in-memory mode" and are not compatible with BucketListDB. Unlike ltx, BucketListDB requires the full ledger close path to update any state. It would be nice to eventually remove this so we could have better test coverage and remove the code overhead from maintaining "in-memory mode." That being said, this is difficult, as many test helper functions assume that arbitrary ledger state can be modified without closing a ledger.