Move gpdtm_plpgsql to different test group

Since it uses serializable backend which is conflicting with
append-optimized tables.

Author: Xin Zhang <xzhang@pivotal.io>
Author: Asim R P <apraveen@pivotal.io>

src/test/regress/greenplum_schedule

@@ -54,7 +54,7 @@ test: cursor
 # so it needs to be in a group by itself
 test: query_finish_pending
 
-test: gpdiffcheck gptokencheck gp_hashagg sequence_gp tidscan co_nestloop_idxscan dml_in_udf
+test: gpdiffcheck gptokencheck gp_hashagg sequence_gp tidscan co_nestloop_idxscan dml_in_udf gpdtm_plpgsql
 
 test: rangefuncs_cdb gp_aggregates gp_dqa subselect_gp subselect_gp2 distributed_transactions olap_group olap_window_seq sirv_functions appendonly create_table_distpol alter_distpol_dropped query_finish
 
@@ -77,7 +77,7 @@ test: sreh
 # ERROR:  parameter "gp_interconnect_type" cannot be set after connection start
 
 ignore: gp_portal_error
-test: external_table external_table_create_privs column_compression compression_zstd eagerfree gpdtm_plpgsql alter_table_aocs alter_table_aocs2 alter_distribution_policy aoco_privileges aocs
+test: external_table external_table_create_privs column_compression compression_zstd eagerfree alter_table_aocs alter_table_aocs2 alter_distribution_policy aoco_privileges aocs
 test: alter_table_set alter_table_gp alter_table_ao ao_create_alter_valid_table subtransaction_visibility oid_consistency udf_exception_blocks
 test: ic
 ignore: icudp_full