tests: clean up a warning on GHC 7.6/7.8
tests: clean up a warning on GHC 7.6/7.8

Signed-off-by: Austin Seipp <7ea35d812706d9213868749011af1ed4fa2f6aa0@well-typed.com>

arianvp/binary-serialise-cbor,arianvp/binary-serialise-cbor