test: check that `flush` doesn't fail all the time
test: check that `flush` doesn't fail all the time

It's hard to check that `flush` works in a simple test, but it's good to
rule out failure modes which would cause it to always fail (such as
flushing on a bad fd)

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

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