fix flaky test: close account before marking thread-upgrade as finished

store/threads_test.go opens an account, starts the threading upgrade, waits for
it to finish, runs some tests, and closes the account at the end, verifying all
references are gone. the "thread upgrade" goroutine has its own account
reference. it closes its account after having signaled completion of the
upgrade. in between that time, all checks from the tests could run, its account
closed and its no-more-account-references check would fail. the fix is
hopefully to mark the thread upgrade process finished after closing the
account. hard to verify, but this only happens very rarely.

store/account.go

@@ -1016,6 +1016,9 @@ func OpenAccountDB(log mlog.Log, accountDir, accountName string) (a *Account, re
 		defer func() {
 			err := closeAccount(acc)
 			log.Check(err, "closing use of account after upgrading account storage for threads", slog.String("account", a.Name))
+
+			// Mark that upgrade has finished, possibly error is indicated in threadsErr.
+			close(acc.threadsCompleted)
 		}()
 
 		defer func() {
@@ -1026,9 +1029,6 @@ func OpenAccountDB(log mlog.Log, accountDir, accountName string) (a *Account, re
 				metrics.PanicInc(metrics.Upgradethreads)
 				acc.threadsErr = fmt.Errorf("panic during upgradeThreads: %v", x)
 			}
-
-			// Mark that upgrade has finished, possibly error is indicated in threadsErr.
-			close(acc.threadsCompleted)
 		}()
 
 		err := upgradeThreads(mox.Shutdown, log, acc, &up)