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@gmail.com>
Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/9ce8f5d8-8ab2-4aa2-b062-c5d74161069c%40gmail.com