test: ensure the ring is empty by using O_TRUNC
test: ensure the ring is empty by using O_TRUNC

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

jonludlam/shared-block-ring,simonjbeaumont/shared-block-ring,djs55/shared-block-ring