(PUP-1588) Constrain new msgpack specs to only run with msgpack lib

When msgpack lib is not present (extra group gems not installed), they
will be ignored.
Merge branch 'issue/master/pup-1588-constrain-new-msgpack-tests'

* issue/master/pup-1588-constrain-new-msgpack-tests:
  (PUP-1588) Constrain new msgpack specs to only run with msgpack lib