add pg_plan_filter tests

nix/tests/expected/pg_plan_filter.out

@@ -0,0 +1,16 @@
+begin;
+  load 'plan_filter';
+  create schema v;
+  -- create a sample table
+  create table v.test_table (
+    id serial primary key,
+    data text
+  );
+  -- insert some test data
+  insert into v.test_table (data)
+  values ('sample1'), ('sample2'), ('sample3');
+  set local plan_filter.statement_cost_limit = 0.001;
+  select * from v.test_table;
+ERROR:  plan cost limit exceeded
+HINT:  The plan for your query shows that it would probably have an excessive run time. This may be due to a logic error in the SQL, or it maybe just a very costly query. Rewrite your query or increase the configuration parameter "plan_filter.statement_cost_limit".
+rollback;

nix/tests/sql/pg_plan_filter.sql

@@ -0,0 +1,22 @@
+begin;
+  load 'plan_filter';
+
+  create schema v;
+
+  -- create a sample table
+  create table v.test_table (
+    id serial primary key,
+    data text
+  );
+
+  -- insert some test data
+  insert into v.test_table (data)
+  values ('sample1'), ('sample2'), ('sample3');
+
+  set local plan_filter.statement_cost_limit = 0.001;
+
+  select * from v.test_table;
+
+rollback;
+
+