test: perform the write-then-read test with interesting buffer sizes
test: perform the write-then-read test with interesting buffer sizes

We know that the protocol does different things depending on the
client's choice of read and write buffer sizes.

This test currently fails.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

samoht/ocaml-vchan,mirage/ocaml-vchan,samoht/ocaml-vchan,mirage/ocaml-vchan,mirage/ocaml-vchan,samoht/ocaml-vchan,djs55/ocaml-vchan,djs55/ocaml-vchan,djs55/ocaml-vchan