pgsql: Stabilize pg_stat_io writes test

Stabilize pg_stat_io writes test

Counting writes only for io_context = 'normal' is unreliable, as backends
using a buffer access strategy could flush all of the dirty buffers out from
under the other backends and checkpointer. Change the test to count writes in
any context. This achieves roughly the same coverage anyway.

Reported-by: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/ZAnWU8WbXEDjrfUE%40telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ef719e7b32fc2ec9f78c2d9de099f7d75b61cd1b

Modified Files
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src/test/regress/expected/stats.out | 8 ++++----
src/test/regress/sql/stats.sql | 9 ++++-----
2 files changed, 8 insertions(+), 9 deletions(-)