Add test for pg_stat_reset_single_table_counters() on index

Add test for pg_stat_reset_single_table_counters() on index

stats.sql is already doing some tests coverage on index statistics, by
retrieving for example idx_scan and friends in pg_stat_all_tables.
pg_stat_reset_single_table_counters() is supported for an index for a
long time, but the case was never covered.

This commit closes the gap, by using this reset function on an index,
cross-checking the contents of pg_stat_all_indexes.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/aN8l182jKxEq1h9f@paquier.xyz