test: use `openfile`, `lseek` and `write` rather than `dd`
test: use `openfile`, `lseek` and `write` rather than `dd`

This is simpler and more portable.

Note that `ftruncate` isn't implemented in Lwt for Win32,
see #42.

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

mirage/mirage-block-unix,samoht/mirage-block-unix,djs55/mirage-block-unix