RE: t/035_standby_logical_decoding.pl might fail on attempt to read wrong timeline

Dear Bertrand,

Thanks for re-posting. Let's focus on the logical decoding stuff.

> > 0004: Add a test for 0003

I found a comment for the test.

```
+# Issue SQL decoding (read_local_xlog_page_guts path) on the pre-connected
+# session.
+$decode_session->query_until(
+ qr/decoding_started/, qq(
+ \\echo decoding_started
+ SELECT count(*) FROM pg_logical_slot_peek_changes('race_slot_sql', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+));
```

Not sure, pg_logical_slot_peek_changes() can be finished without waiting anything,
right? So why do we use query_until() and check the output here?
IIUC, the command can immediately raise an ERROR if the race happened. So isn't it
enough to use safe_psql() to ensure the SQL function can work?

Best regards,
Hayato Kuroda
FUJITSU LIMITED