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