pgsql: Add index scan pgstat test coverage.

Add index scan pgstat test coverage.

Add tests that verify the exact values of the idx_tup_read and
idx_tup_fetch counters in pg_stat_all_indexes. There was no existing
coverage of pg_stat_all_indexes, beyond some counter reset tests;
idx_tup_read and idx_tup_fetch had no tests at all.

This is preparation for an upcoming commit that will move index scan
heap fetches behind a new slot-based table AM interface, relocating the
code that maintains these counters.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Reviewed-by: Tomas Vondra <tomas(at)vondra(dot)me>
Discussion: https://postgr.es/m/CAH2-Wz%3DV0V5WGuo85D-OF%2BTM_CxU3q%3DMMkY-ebY0K8xc-9Xykw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/83032dc42cab67dfdd33152d08434785977f069f

Modified Files
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src/test/regress/expected/stats.out | 62 +++++++++++++++++++++++++++++++++++++
src/test/regress/sql/stats.sql | 32 +++++++++++++++++++
2 files changed, 94 insertions(+)