pgsql: Fix flakey pg_stat_io test

Fix flakey pg_stat_io test

Wrap test of pg_stat_io's tracking of shared buffer reads in a transaction to
prevent concurrent accesses (e.g. by autovacuum) causing spurious test
failures.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://www.postgresql.org/message-id/20230306190919.ai6mxdq3sygyyths%40awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1be0fdb9de559ce8daf6d5603f0329cbd056111a

Modified Files
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src/test/regress/expected/stats.out | 4 ++++
src/test/regress/sql/stats.sql      | 4 ++++
2 files changed, 8 insertions(+)