The unit test is no longer valid after SERVER-42772, because now the coordinator is removed from the catalog asynchronously.

However, we could add some more test coverage:

• That a coordinator is removed from the catalog on completion. From a quick search, I only found one unit test of this, and it is for when the coordinator is canceled. We could add coverage for the coordinator completing successfully or with an error.
• That calling TransactionCoordinatorService::coordinateCommit for a lsid and txnNumber whose coordinator has completed either returns boost::none (because the coordinator has already been removed) or a ready future (the coordinator has yet to be removed).