Fix flaky test in multi_foreign_key_relation_graph (#4476)

CREATE TABLE does not invalidate foreign key graph but some other set of
ddl commands do.

Previously, as we run multi_foreign_key & multi_foreign_key_relation_graph
in parallel, it's possible that multi_foreign_key invalidates foreign key
graph via some ddl commands and create table test in
multi_foreign_key_relation_graph becomes flaky.

So we un-parallelize those two tests.

src/test/regress/multi_schedule

@@ -302,7 +302,8 @@ test: node_conninfo_reload
 # ----------
 # multi_foreign_key tests foreign key push down on distributed tables
 # ----------
-test: multi_foreign_key multi_foreign_key_relation_graph
+test: multi_foreign_key
+test: multi_foreign_key_relation_graph
 
 # ----------
 # multi_replicate_reference_table tests replicating reference tables to new nodes after we add new nodes