pgsql: pgstat: add/extend tests for resetting various kinds of stats.

pgstat: add/extend tests for resetting various kinds of stats.

- subscriber stats reset path was untested
- slot stat sreset path for all slots was untested
- pg_stat_database.sessions etc was untested
- pg_stat_reset_shared() was untested, for any kind of shared stats
- pg_stat_reset() was untested

Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Author: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5264add7847871d61d36a5770dac2139d6a7bc80

Modified Files
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contrib/test_decoding/expected/stats.out | 117 +++++++---
contrib/test_decoding/sql/stats.sql | 32 ++-
src/test/recovery/t/006_logical_decoding.pl | 63 ++++++
src/test/regress/expected/stats.out | 164 ++++++++++++++
src/test/regress/parallel_schedule | 3 +
src/test/regress/sql/stats.sql | 78 +++++++
src/test/subscription/t/026_stats.pl | 318 +++++++++++++++++++++-------
7 files changed, 664 insertions(+), 111 deletions(-)