Fill out compatibility test suite

[cdata]

As a Polymer element author, when I implement a carbon-storage-based element, I want a test suite to validate my implementation, so that I know it is consistent with other working carbon-storage elements.

Storage layers all have unique semantics and constraints related to how data changes over time, and this makes it easier than it might otherwise be to create inconsistent implementations. The compatibility test suite exists so that we can reduce or eliminate inconsistencies between various carbon-storage-based elements.