Fix flaky validate_constraint test (#7293)

Sometimes validate constraint would fail like this:

```diff
  validatable_constraint_8000016 | t
 (10 rows)

 DROP TABLE constrained_table;
+ERROR:  deadlock detected
+DETAIL:  Process 16602 waits for ShareRowExclusiveLock on relation 56258 of database 16384; blocked by process 16601.
+Process 16601 waits for AccessShareLock on relation 56120 of database 16384; blocked by process 16602.
+HINT:  See server log for query details.
 DROP TABLE referenced_table CASCADE;
 DROP TABLE referencing_table;
 DROP SCHEMA validate_constraint CASCADE;
-NOTICE:  drop cascades to 3 other objects
+NOTICE:  drop cascades to 4 other objects
 DETAIL:  drop cascades to type constraint_validity
 drop cascades to view constraint_validations_in_workers
 drop cascades to view constraint_validations
+drop cascades to table constrained_table
 SET search_path TO DEFAULT;

```

Source:
https://github.com/citusdata/citus/actions/runs/6708383699?pr=7291

This change fixes that by not running together with the
foreign_key_to_reference_table test anymore. In passing it also
simplifies dropping of the test its resources.

src/test/regress/expected/validate_constraint.out

@@ -133,12 +133,6 @@ ORDER BY 1, 2;
  validatable_constraint_8000016 | t
 (10 rows)
 
-DROP TABLE constrained_table;
-DROP TABLE referenced_table CASCADE;
-DROP TABLE referencing_table;
+SET client_min_messages TO WARNING;
 DROP SCHEMA validate_constraint CASCADE;
-NOTICE:  drop cascades to 3 other objects
-DETAIL:  drop cascades to type constraint_validity
-drop cascades to view constraint_validations_in_workers
-drop cascades to view constraint_validations
 SET search_path TO DEFAULT;

src/test/regress/multi_1_schedule

@@ -201,7 +201,8 @@ test: citus_copy_shard_placement
 # multi_utilities cannot be run in parallel with other tests because it checks
 # global locks
 test: multi_utilities
-test: foreign_key_to_reference_table validate_constraint
+test: foreign_key_to_reference_table
+test: validate_constraint
 test: multi_repartition_udt multi_repartitioned_subquery_udf multi_subtransactions
 
 test: multi_modifying_xacts

src/test/regress/multi_schedule_hyperscale

@@ -154,7 +154,8 @@ test: multi_outer_join
 # ---
 test: multi_complex_count_distinct
 test: multi_upsert multi_simple_queries
-test: foreign_key_to_reference_table validate_constraint
+test: foreign_key_to_reference_table
+test: validate_constraint
 
 # ---------
 #  creates hash and range-partitioned tables and performs COPY

src/test/regress/multi_schedule_hyperscale_superuser

@@ -150,7 +150,9 @@ test: multi_outer_join
 test: multi_create_fdw
 test: multi_generate_ddl_commands multi_create_shards multi_prune_shard_list
 test: multi_upsert multi_simple_queries multi_data_types
-test: multi_utilities foreign_key_to_reference_table validate_constraint
+test: multi_utilities
+test: foreign_key_to_reference_table
+test: validate_constraint
 test: multi_repartition_udt multi_repartitioned_subquery_udf
 
 # ---------

src/test/regress/sql/validate_constraint.sql

@@ -116,9 +116,6 @@ SELECT *
 FROM constraint_validations_in_workers
 ORDER BY 1, 2;
 
-DROP TABLE constrained_table;
-DROP TABLE referenced_table CASCADE;
-DROP TABLE referencing_table;
-
+SET client_min_messages TO WARNING;
 DROP SCHEMA validate_constraint CASCADE;
 SET search_path TO DEFAULT;