pgsql: 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(dot)pg(at)gmail(dot)com>
Discussion: https://postgr.es/m/aN8l182jKxEq1h9f@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c173aaff98f898564998c0f5e4df9b6fca8a0f9c

Modified Files
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src/test/regress/expected/stats.out | 21 +++++++++++++++++++++
src/test/regress/sql/stats.sql | 9 +++++++++
2 files changed, 30 insertions(+)