pg_walsummary: Improve stability of test checking statistics

Per buildfarm member culicidae, the query checking for stats reported by
the WAL summarizer related to WAL reads is proving to be unstable.

Instead of a one-time query, this commit replaces the logic with a
polling query checking for the WAL read stats, making the test more
reliable on machines that could be slow with the stats reports.

This test has been introduced in f4694e0f35b2, so backpatch down to v18.

Reported-by: Alexander Lakhin <[email protected]>
Reviewed-by: Alexander Lakhin <[email protected]>
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 18

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