pg_stat_statements: Fix test instability with cache-clobbering builds

Builds with CLOBBER_CACHE_ALWAYS enabled are failing the new test
introduced in 1572ea96e6, checking the nesting level calculation in
the planner hook.  The inner query of the function called twice is
registered as normalized, as such builds would register a PGSS entry in
the post-parse-analyse hook due to the cached plans requiring
revalidation.

A trick based on debug_discard_caches cannot work as far as I can, a
normalized query still being registered.  This commit takes a different
approach with the addition of a DISCARD PLANS before the first function
call.  This forces the use of a normalized query in the PGSS entry for
the inner query of the function with and without CLOBBER_CACHE_ALWAYS,
which should be enough to stabilize the test.  Note that the test is
still checking what it should: when removing the nesting level
calculation in the planner hook of PGSS, one still gets a failure for
the PGSS entry of the inner query in the function, with "toplevel" being
flipped to true instead of false (it should be false, as a non-top-level
entry).

Per buildfarm members avocet and trilobite, at least.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/82dd02bb-4e0f-40ad-a60b-baa1763ff0bd@gmail.com

contrib/pg_stat_statements/expected/level_tracking.out

@@ -1543,6 +1543,13 @@ SELECT pg_stat_statements_reset() IS NOT NULL AS t;
 
 -- planner - all-level tracking.
 SET pg_stat_statements.track_planning = TRUE;
+-- Release all cached plans before the first function call.  This matters
+-- when debug_discard_caches is enabled, which would store a normalized
+-- version of the inner query of the function.  Forcing a plan rebuild
+-- ensures that a normalized version is always stored with the stats entry,
+-- while checking that the nesting level is computed correctly in the
+-- planner hook.
+DISCARD PLANS;
 SELECT PLUS_THREE(8);
  plus_three 
 ------------
@@ -1560,7 +1567,7 @@ SELECT toplevel, calls, rows, plans, query FROM pg_stat_statements
  toplevel | calls | rows | plans |                               query                                
 ----------+-------+------+-------+--------------------------------------------------------------------
  t        |     2 |    2 |     2 | SELECT PLUS_THREE($1)
- f        |     2 |    2 |     2 | SELECT i + 3 LIMIT 1
+ f        |     2 |    2 |     2 | SELECT i + $2 LIMIT $3
  t        |     1 |    1 |     0 | SELECT pg_stat_statements_reset() IS NOT NULL AS t
  t        |     0 |    0 |     1 | SELECT toplevel, calls, rows, plans, query FROM pg_stat_statements+
           |       |      |       |   ORDER BY query COLLATE "C"

contrib/pg_stat_statements/sql/level_tracking.sql

@@ -435,7 +435,13 @@ SELECT pg_stat_statements_reset() IS NOT NULL AS t;
 
 -- planner - all-level tracking.
 SET pg_stat_statements.track_planning = TRUE;
-
+-- Release all cached plans before the first function call.  This matters
+-- when debug_discard_caches is enabled, which would store a normalized
+-- version of the inner query of the function.  Forcing a plan rebuild
+-- ensures that a normalized version is always stored with the stats entry,
+-- while checking that the nesting level is computed correctly in the
+-- planner hook.
+DISCARD PLANS;
 SELECT PLUS_THREE(8);
 SELECT PLUS_THREE(10);