stats test intermittent failure

Hi,

Jeff pointed out that one of the pg_stat_io tests has failed a few times
over the past months (here on morepork [1] and more recently here on
francolin [2]).

Failing test diff for those who prefer not to scroll:

+++ /home/bf/bf-build/francolin/HEAD/pgsql.build/testrun/recovery/027_stream_regress/data/results/stats.out
2023-07-07 18:48:25.976313231 +0000
@@ -1415,7 +1415,7 @@
:io_sum_vac_strategy_after_reuses > :io_sum_vac_strategy_before_reuses;
?column? | ?column?
----------+----------
- t | t
+ t | f

My theory about the test failure is that, when there is enough demand
for shared buffers, the flapping test fails because it expects buffer
access strategy *reuses* and concurrent queries already flushed those
buffers before they could be reused. Attached is a patch which I think
will fix the test while keeping some code coverage. If we count
evictions and reuses together, those should have increased.

- Melanie

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=morepork&dt=2023-06-16%2018%3A30%3A32
[2] https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=francolin&dt=2023-07-07%2018%3A43%3A57&stg=recovery-check