test: check we can read back patterns we write
test: check we can read back patterns we write

This is a very simple test which writes sectors full of byte x to
sector x for the first 255 sectors, and then reads them back again.

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

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