Improve test portability of plpgsql scenario

This test expects query IDs to be computed without the hint table,
checking that compute_query_id is either "on" or "auto" when running the
tests.  The test would fail should compute_query_id be set to off on a
cluster where the regression tests are run.

There are no reasons why the test should not just use compute_query_id =
on now that we don't depend on pg_stat_statements anymore since
8e296b6 for the regression tests of pg_hint_plan.  So let's just
force it, improving the portability of the tests.

Backpatch-through: 17

expected/plpgsql.out

@@ -6,12 +6,7 @@ SET client_min_messages TO log;
 \set SHOW_CONTEXT always
 LOAD 'pg_hint_plan';
 SET pg_hint_plan.debug_print TO on;
-SELECT setting <> 'off' FROM pg_settings WHERE name = 'compute_query_id';
- ?column? 
-----------
- t
-(1 row)
-
+SET compute_query_id = on;
 SHOW pg_hint_plan.enable_hint_table;
  pg_hint_plan.enable_hint_table 
 --------------------------------

sql/plpgsql.sql

@@ -8,7 +8,7 @@ SET client_min_messages TO log;
 
 LOAD 'pg_hint_plan';
 SET pg_hint_plan.debug_print TO on;
-SELECT setting <> 'off' FROM pg_settings WHERE name = 'compute_query_id';
+SET compute_query_id = on;
 SHOW pg_hint_plan.enable_hint_table;
 
 -- Internal handling of hints within plpgsql functions.