Add regression test vectors for cached data

[jqmp]

When we change Bionic's caching code, we run the risk of making backwards-incompatible changes, creating a situation where the current version of Bionic can't load cached data from older versions. A recent example of this is the bug fixed here. Our current tests can't catch these errors because they both save and load data using the latest code.

One way to catch these errors automatically is to write a set of tests that attempt to load data from a fixed, checked-in set of cached files. This will force us to explicitly update these files when we make a non-backwards-compatible change. (If we do make such a change, we should bump the CACHE_SCHEMA_VERSION constant -- to avoid trying to load the old files at all -- and the minor version number.)