Adding another test validated from the FreeBSD workflow.
Just because the keywords requested to be validated are not present in
the manifest, it is not an error.
Also, if the keywords from a new manifest are not present in a prior
manifest, then only compare the common keywords.
Fixes https://github.com/vbatts/go-mtree/issues/86
Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>
