Broken snapshot test

The test confirming that deleting sft files wer eheld open whilst
snapshots were registered was actually broken.  This test has now been
fixed, as well as the logic in registring snapshots which had used
ledger_sqn mistakenly rather than manifest_sqn.

src/leveled_pclerk.erl

@@ -118,8 +118,8 @@ handle_call({manifest_change, confirm, Closing}, From, State) ->
             io:format("Prompted confirmation of manifest change~n"),
             gen_server:reply(From, ok),
             WI = State#state.work_item,
-            mark_for_delete(WI#penciller_work.unreferenced_files,
-                            State#state.owner),
+            ok = mark_for_delete(WI#penciller_work.unreferenced_files,
+                                    State#state.owner),
             {noreply,
                 State#state{work_item=null, change_pending=false},
                 ?MIN_TIMEOUT}