pg_stat_statements: Add more tests with temp tables and namespaces

pg_stat_statements: Add more tests with temp tables and namespaces

These tests provide coverage for RangeTblEntry and how query jumbling
works with search_path, as well as the case where relations are
re-created, generating a different query ID as the relation OID is used
in the computation.

A patch is under discussion to switch to a different approach based on
the relation name, and there was no test coverage for this area,
including how queries are currently grouped with search_path.  This is
useful to track how the situation changes between HEAD and any patches
proposed.

Christoph has proposed the test with ON COMMIT DROP temporary tables,
and I have written the second part.

Author: Christoph Berg 
Author: Michael Paquier 
Discussion: https://postgr.es/m/[email protected]