test/read-write: add more buffer select tests
test/read-write: add more buffer select tests

Test both vectored and non-vectored reads, and also add two test
cases for vectored and non-vectored short reads.

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing