Add spec to exercise creating snapshots against a baseline

In 5e382ae4555f5b0866 we fixed a bug that broke snapshots being created
against a baseline. This commit makes our code safer by adding a spec to
prevent this from happening again.

This also introduces the :accepted trait on the Snapshot factory.

Change-Id: Id0cdc708c2f61a1e540351399fb0a1dbe2e722e3