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