pgsql: aio: Add test_aio module

aio: Add test_aio module

To make the tests possible, a few functions from bufmgr.c/localbuf.c had to be
exported, via buf_internals.h.

Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Co-authored-by: Andres Freund <andres(at)anarazel(dot)de>
Co-authored-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/93bc3d75d8e1aabdc256ff6da2282266dca82537

Modified Files
--------------
src/backend/storage/buffer/bufmgr.c | 8 +-
src/backend/storage/buffer/localbuf.c | 3 +-
src/include/storage/buf_internals.h | 7 +
src/test/modules/Makefile | 1 +
src/test/modules/meson.build | 1 +
src/test/modules/test_aio/.gitignore | 2 +
src/test/modules/test_aio/Makefile | 26 +
src/test/modules/test_aio/meson.build | 37 +
src/test/modules/test_aio/t/001_aio.pl | 1503 +++++++++++++++++++++++++
src/test/modules/test_aio/t/002_io_workers.pl | 125 ++
src/test/modules/test_aio/test_aio--1.0.sql | 108 ++
src/test/modules/test_aio/test_aio.c | 806 +++++++++++++
src/test/modules/test_aio/test_aio.control | 3 +
13 files changed, 2622 insertions(+), 8 deletions(-)