Fixe test for partitioned table snapshot

Postgresql JDBC driver >= 42.1.2 introduced support for partitioned tables in JDBC - https://github.com/pgjdbc/pgjdbc/pull/823/files

It is thus necessary to update the snapshot test as now the recpords come both from partitioned table and its partitions. The records from partitioned table do not have key as the table itself doesn't have primary key defined.