Add test case for minimum length left for incremental buffers

See the link for more details, but this adds support for the application
to tell the kernel what the minimum size left in an incrementally
consumed buffer should be to consider it valid, rather than just assume
that even 1 byte is enough for that.

Link: https://github.com/axboe/liburing/issues/1433
Signed-off-by: Jens Axboe <axboe@kernel.dk>