pgsql: pg_stat_statements: Add coverage for entry_dealloc()

pg_stat_statements: Add coverage for entry_dealloc()

This involves creating more than pg_stat_statements.max entries and
checking that the limit is kept and the least used entries are kicked
out.

Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Julien Rouhaud <rjuju123(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/40d1e4f2-835f-448f-a541-8ff5db75bf3d(at)eisentraut(dot)org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/742f6b3e6df980d7dafa4a18a165d285483d5f0e

Modified Files
--------------
contrib/pg_stat_statements/Makefile | 2 +-
contrib/pg_stat_statements/expected/max.out | 82 ++++++++++++++++++++++
contrib/pg_stat_statements/meson.build | 1 +
contrib/pg_stat_statements/pg_stat_statements.conf | 2 +
contrib/pg_stat_statements/sql/max.sql | 61 ++++++++++++++++
5 files changed, 147 insertions(+), 1 deletion(-)