fix message size in a message in gentestdata

and work around the message in test-upgrade.sh.
and add subcommand to open an account, triggering data upgrades.

gentestdata.go

@@ -356,7 +356,7 @@ Accounts:
 			MailboxDestinedID: sent.ID,
 			Flags:             store.Flags{Seen: true, Junk: true},
 			Size:              int64(len(prefix1) + len(msg1)),
-			MsgPrefix:         []byte(prefix),
+			MsgPrefix:         []byte(prefix1),
 		}
 		mf1 := tempfile()
 		xcheckf(err, "creating temp file for delivery")