[PATCH] TAP test showing that pg_replication_slot_advance() works on standby

Hi all

The attached patch adds a test to the TAP logical decoding suite to show
that pg_replication_slot_advance() works on a server in standby mode.

I didn't add a full demonstration of how to do failover with logical slots
because we're still missing a way to "sync" a logical slot from a primary
to a standby, or a way to directly create such a slot safely on a standby
in a way that enforces a safe catalog_xmin etc.

You can't replay from the slot unless the server is promoted, so I don't
test that.

I'm not sure if anyone's going to find it worth committing, but it's here
so searchers can find it at least.