Add pg_dump tests for CREATE STATISTICS
Add pg_dump tests for CREATE STATISTICS

CREATE STATISTICS pg_dump support code was not covered at all by
previous tests.

Discussion: 3b961c8c210aec51619245e07b76d5cbda923d80@alvherre.pgsql

xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb