Add basic SnapshotWorker spec

In 3a14ea2c64d6f5 I added some special behavior for SnapshotWorkers that
prevents them from becoming zombies if their Snapshots no longer exist
by the time they are executed. This commit covers that behavior with a
spec.

Change-Id: Id175813b355180a1ce7b68331b4ce8289d2d7a22