pgsql: pg_stat_statements: Fix test instability with cache-clobbering b

pg_stat_statements: Fix test instability with cache-clobbering builds

Builds with CLOBBER_CACHE_ALWAYS enabled are failing the new test
introduced in 1572ea96e657, 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(at)gmail(dot)com>
Discussion: https://postgr.es/m/82dd02bb-4e0f-40ad-a60b-baa1763ff0bd@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/72e3abd082c3ae6db4081137a7431fdd05e55d73

Modified Files
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contrib/pg_stat_statements/expected/level_tracking.out | 9 ++++++++-
contrib/pg_stat_statements/sql/level_tracking.sql | 8 +++++++-
2 files changed, 15 insertions(+), 2 deletions(-)