pgsql: Fix test_aio read_buffers() to work without cassert

Fix test_aio read_buffers() to work without cassert

In a production build, StartReadBuffers() doesn't populate all fields
of a ReadBuffersOperation for a buffer hit because no callers use them
(they are populated in assert builds).

read_buffers() (a test-only function) relied on some of these fields, so
AIO tests failed on non-assert builds (discovered on the buildfarm after
commit 020c02bd908).

Fix by tracking the required information ourselves in read_buffers() and
avoiding reliance on the ReadBuffersOperation unless we know that we did
IO.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/9ce8f5d8-8ab2-4aa2-b062-c5d74161069c%40gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/8519251ee975a8cca16fb35d699c1a39f2d217a1

Modified Files
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src/test/modules/test_aio/test_aio.c | 22 ++++++++++------------
1 file changed, 10 insertions(+), 12 deletions(-)