Fix test_decoding test case's check that slot has been dropped.

pg_stat_replication shows connected replication clients. The ddl test case
never has any replication clients connected, so querying pg_stat_replication
is pointless. To check that a slot has been dropped correctly, query
pg_replication_slots instead.

Andres Freund
Add test case for logical decoding of prepared transactions.

Andres Freund